A ZODB database can be changed by local client as well as another
process. A notification channel is thus needed for local cache and
database view to know they have to update to current database state.

This patch builds the interface of how such notifications should be
provided by drivers. Contrary to ZODB/py it is required that every
driver provide it.

However we will be providing driver support incrementally and for now
all drivers behave as if the database is not changing.

A note on why Watchq is passed to driver as options: low-level ZODB
users, who might want to work with drivers directly, might not need it,
and this way with Watchq not present in driver options they will continue to
observe the same driver behaviour as before watchq was introduced. In
practice many low-level utilities don't need notification support, and
it would be not good to required them all to update open calls and to
provide watchq drainer not to get stuck.

TODO Watchq should be extended to also receive errors from watcher, so
that clients could be notified when there is something wrong with the
database.