gets embedded v8 compiling on 3.19
 is  in javascript - ROTS by @coffeemug
get's everything running too
adds ifdef guards for v8 version
fix typo as per code review
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
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
removing unused mailbox_callback_mode_t and mailbox_manager_t::check_existence
Check for v8 version in configure script
Merge branch 'v1.7.x' into 1195_v8_update
Merge branch 'v1.7.x' into next