It is pointless for lastTransaction() to block until it is allowed to
return the TID of a transaction that has just been committed, because
it may still not be the real last TID (e.g. for some storage
implementations, invalidations are received from a shared server
via the network). While invalidations are still being processed,
it's fine to return immediately with the previous last TID.

This was clarified in commit 4a6b0283
("mvccadapter: check if the last TID changed without invalidation").

See pull request #316