s4-ldb: Implement ldb_msg_difference() function to accept a memory context from client

s4-ldb: Implement ldb_msg_difference() function to accept a memory context from client

Old implementation from ldb_msg_diff() was moved into
this this function but with changed interface
so that a memory context may be passed.

ldb_msg_diff() function is now based on ldb_msg_difference(),
which fixes a hidden leak - internal ldb_msg object
(returned from ldb_msg_canonicalize) wasn't freed
and stays attached to ldb_context for the connection lifetime.