Merge branch 'master' into t

* master:
  go/zodb/btree: New package to work with ZODB BTrees (draft)
  go/zodb: DB - application-level handle to database (very draft)
  go/zodb: Connection: Allow applications to tune live-cache eviction policy
  go/zodb: Implement Connection
  go/zodb/internal/weak: New package to handle weak references
  go/zodb: PyStateful persistency support
  go/zodb: pydata: Factor out class extraction logic into xpyclass
  go/zodb: Persistent - the base type to implement IPersistent objects
  go/zodb: IPersistent + Connection stub
  go/zodb: pkgdoc: Add section overviewing storage drivers
  go/zodb: pkgdoc: Put zodbtools reference into "Miscellaneous" section
  go/zodb: pkgdoc: "Operations" -> "Storage layer"
  go/zodb: pkgdoc: Stress that objects can reference each other in the database
  go/zodb: Pkgdoc cosmetics
  go/zodb: Provide invalid Oid value
  go/zodb: OpenOptions: Document that NoCache disables prefetch
  go/transaction: New package to deal with transactions (very draft)