reworked mailboxes, creation and destruction is now slower, but they will no longer block the connectivity cluster while receiving, added a coro_pool to parallel traversal to limit coroutine explosions
Merge branch 'next' into grey_issue_1169
undid some of the mailbox changes, modified message handlers to take a string-based stream rather than a generic stream, which allows for some more efficient usage
Merge branch 'next' into grey_issue_1169
removing unused mailbox_callback_mode_t and mailbox_manager_t::check_existence
Merge branch 'next' into grey_issue_1169
Merge branch 'next' into grey_issue_1169