PREVIEW: alter ldb_msg_canonicalize to allocate memory in NULL

PREVIEW: alter ldb_msg_canonicalize to allocate memory in NULL

Returned message will be allocated into NULL memory
context. It is strongly advised for callers to steal
the message returned into a local memory context.
Allocating ldb_msg into NULL context should appear
as memory leak (kind of) and thus make it easier to identify
places where ldb_msg_canonicalize() is misused