ctdb-protocol: Replace ctdb_event_header with sock_packet_header

ctdb-protocol: Replace ctdb_event_header with sock_packet_header

This removes the static declaration and adds prototype declarations
of ctdb_event_header marshalling functions to avoid compiler warnings.
These functions will be removed.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>