PQueue.clear: if the node doesn't exist in the store then don't fail

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
test: clear any persisted watches between 'connections'

In a real server the flusher thread should transmit the watch events
and clear the queue. In the simulation, we clear the queue on 'connect'
since we don't actually have a flusher thread.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>