ovsdb: retry calls in case of communication error with server

When the server is restarted the write to unix socket fails with EPIPE. In such case, don't fail all the calls in queue; instead, after a sync of the ovsdb state (through a monitor call), start processing the queue again, including the call that previously failed.

Add a retry counter to avoid that calls are stuck in the queue forever in a hypothetical scenario in which the write always fails.