Made db_term_t use the cluster_access.databases_semilattice_metadata watchable.
Replaced other thread-crossing use of db_searcher, removed db_searcher from const_rethreading_metadata_accessor_t.
Removed unnecessarily cross-threaded const_rethreading_metadata_accessor_t::ns_searcher uses.
Removed comment about semilattice_metadata.
Merge branch 'next' into sam_1907
Merge branch 'sam_1907' into next