DBObjectMap: restructure map_header_in_use locking

Tieing map_header_in_use to a _Header is a mistake since ownership of
the map_header_in_use can move (clone).  Instead, grab the
map_header_in_use entry at the top of each call chain and release at the
end.  To facilitate that, we introduce a MapHeaderLock object and
require that it be passed to the various map header manipulation
methods.

Fixes: #9326
Signed-off-by: Samuel Just <sam.just@inktank.com>