XID cache is now based on lock-free hash.
Also fixed lf_hash_destroy() to call alloc destructor.
Note that previous implementation had race condition when thread was accessing
XA owned by different thread. This new implementation doesn't fix it either.
Fix a possible race in the test case when restarting the server.
Make sure we have disconnected before waiting for the reconnect
that signals that the server is back up. Otherwise, we may in
rare cases continue the test while the old server is shutting
down, eventually leading to "connection lost" failure.
