s4-ldbwrap: added re-use of ldb contexts in ldb_wrap_connect()

s4-ldbwrap: added re-use of ldb contexts in ldb_wrap_connect()

This allows us to reuse a ldb context if it is open twice, instead
of going through the expensive process of a full ldb open. We can
reuse it if all of the parameters are the same.

The change relies on callers using talloc_unlink() or free of a parent
to close a ldb context.