When the clear timeout of the cache is zero, we launch a thread to synchronize
caches if the back-end support channels.
The thread is started on the first call to sync for the database.
The channel is also used to clear the cache of the process initiating the
clearing. This allows to test the mechanism is working with a single process.
So there will be a small delay even on single instance.
issue8027
review68431002
We implement sync, commit and rollback to follow transaction semantic.
When a transaction clears a cache, the global cache is cleared only when the
transaction is committed meanwhile the transaction will use a cache for itself
only.
We remove _nocache option because the sync, commit and rollback operate only
for the transaction.
issue7975
review53521002
Cédric Krier