map: When removing a key, don't NULL the entry, but mark as deleted.

When searching next time, such entry should be just skipped, not terminate
the search. It's known that marking techique is not efficient at the presense
of many removes, but namespace usage should not require many deletes, and
as for user dictionaries - well, open addressing map table with linear
rehashing and load factor of ~1 is not particularly efficient at all ;-).
TODO: May consider "shift other entries in cluster" approach as an
alternative.