* master:
  go/zodb: Provide String for events
  go/zodb/*: Cosmetics

This helps when events are printed and logged.

* master:
  go/zodb/*: Fix Python database generators to work with ZODB5 + preserve DB compatibility with ZODB3

Paralleling nexedi/zodbtools@0b6f99da.

All test databases in */testdata/ stay as before bit-to-bit the same.

* master: (79 commits)
  go/zodb/zodbtools: Regenerate test data + fix
  go/zodb/fs1: Regenerate
  go/neo/proto: Regenerate
  go/neo/proto: Fix protogen to work without prior `go install`
  go/*: Add Unwrap to custom errors that have Cause
  go/zodb/btree: Represent a tree node - either BTree or Bucket - by Node
  go/zodb: Cosmetics
  go/zodb: LiveCache: Reclassify all objects on cache control change
  go/zodb: LiveCache: Allow objects to be pinned / omitted from the cache
  go/zodb: Prepare to rework/fix LiveCache control
  go/zodb: persistent tests: Factor test database setup out of TestPersistentDB
  go/zodb: persistent test: Fix thinko in tPersistentDB.checkObj
  go/zodb: Don't expose LiveCache internal organization onto its users
  go/zodb: OpenStorage -> Open
  go/zodb: LastTid -> Sync + Head
  go/zodb: DB: preserve δtail not to go down to ø slice on seldom commits
  go/zodb: ΔTail += Data()
  go/zodb/*: Cosmetics, typos, ...
  go/zodb: Connection.Resync
  go/zodb: DB: handle invalidations
  ...

The only change is extra LF that is appended to end of 1.zdump.pyok due
to nexedi/zodbtools@624aeb09.

Correspondingly fix dump on zodbtools/go side.

The only update comes from

b@8c9698d6

Rerun `go generate` for this package. Updates come due to updated
`stringer`. zproto-marshal.go stays the same.

When trying to regenerate zproto-marshal.go on a fresh go workspace I
got:

    (neo) (z-dev) (g.env) kirr@deco:~/src/neo/src/lab.nexedi.com/kirr/neo/go/neo/proto$ go generate
    typecheck: ../../zodb/zodb.go:174:2: could not import lab.nexedi.com/kirr/go123/mem (can't find import: "lab.nexedi.com/kirr/go123/mem")
    exit status 1
    proto.go:57: running "sh": exit status 1

Digging around a bit this turned out to be due to that
importer.Default() is using packages in only binary installed form:

    https://github.com/golang/go/issues/19334#issuecomment-283430322
    https://github.com/golang/go/issues/11415

Fix it by using "source" importer which always looks at sources.

(zproto-marshal.go stays unchanged after `go generate` rerun)

So that Go1.13 and golang.org/x/xerrors understand the wrapping:
https://blog.golang.org/go1.13-errors

Preserve Cause for compatibility with github.com/pkg/errors.

This allows to use proper btree.Node type - not just zodb.IPersistent -
in places which expects a tree node.

More comments, typos, ...

If cache control changes, all objects needs to go through new cache
control as what was e.g. pinned before could be in other class
from the point of view of new cache control.

Tests pending...

We already have LiveCacheControl and policy to keep object state in live
cache. However that state is currently kept only until the object is
present in the cache, and the object is there currently only until there
are live pointers to the object from anywhere. If all such pointers go
away, LiveCache currently discards the object.

Add a way (PCachePinObject) to tell live cache that it must retain an
object even if there are no other pointers to it. This is needed for
Wendelin.core which semantically relies on some objects to be present in
live cache, even if in ghost state, for propagating ZODB invalidations
into OS pagecache:

https://lab.nexedi.com/kirr/wendelin.core/blob/000bf16359/wcfs/wcfs.go#L245

For symmetry add a way (PCacheDropObject) to tell live cache that it
must not retain an object. This makes sense for objects that are read in
large only once to avoid needlessly evicting all other objects while
making the room for what won't be soon used again.

For completeness add also a way (PCacheDropState) to tell live cache
that the state of this object should not be cached. This can make sense
even for objects that are pinned: for example Wendelin.core pins ZBlk
objects to rely on volatile metadata it attaches to them, but have to
drop ZBlk state that was loaded from database: that state is propagated
to be cached in OS pagecache, and keeping it also in ZODB live cache
would just waste RAM.

Finally add test to cover LiveCache/LiveCacheControl functionality in
detail.

We are going to rework LiveCache to support both retaining objects in
cache even though if all pointers to them went away (currently such
objects disappear from cache), and to tell the cache that such and such
object should not be cached at all, e.g. not to evict all other objects
on one-time large reads.

This will be done by LiveCacheControl giving cache policy classification
for an object. Prepare for that and change

	WantEvict(obj) -> bool

to

	PCacheClassify(obj) PCachePolicy

which can give more information to LiveCache about how to handle an
object.

This is only a preparatory patch as there is neither no new
functionality nor fixes here.

We already have "testing connection" functionality in common place (was
named tPersistentDB -> now tConnection). However the functionality to
"setup and use testing database" was inside TestPersistentDB only.

Since we are going to add more tests, factor that functionality out of
TestPersistentDB into shared infrastructure that could be reused in
between tests (-> tDB).

It was printing oid instead of obj.

Provide functions to:

- set cache element
- iterate through all elements

LiveCache organization will be changed in the following patch, and it is
better to keep the knowledge about its internal structure localized.