Fix for re-sync logic in reading packet from backend.

Fix for re-sync logic in reading packet from backend.

read_kind_from_backend(), which reads message kind from backend,
re-syncs backend nodes when a ready for query message is
received. Unfortunately it forgot to call
pool_pending_message_pull_out() to delete sync pending message. This
leads to random stuck while reading packets from backend. Fix this to
call pool_pending_message_pull_out().