Move some common functions to common/ctdb_ltdb.c

Move some common functions to common/ctdb_ltdb.c

Move identical copies of ctdb_null_func(), ctdb_fetch_func(),
ctdb_fetch_with_header_func() from ctdb_client.c and
ctdb_ltdb_server.c to somewhere common.

This is in the context of wanting to run CCAN-style tests where most
of the ctdbd code is just included in the test program.

Signed-off-by: Martin Schwenke <martin@meltin.net>