Removed a false const_cast.
Added rdb_context_t::get_namespaces_metadata, get_databases_metadata, not used yet.
Removed external exposure of watchables by env_t.
Made env_t::join_and_wait_to_propagate, moved from cluster_access_t.
Removed a pointless boost function intermediary.
Made join_and_wait_to_propagate assert about which thread it is on.
Made RDBInterrupt tests run correctly.