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
Cédric Krier
e5e92ac81a