adds ability to kill procs
Added /gdb_restart

There's a timing issue where sometimes the server gets messed up
if you restart gdb too many times in quick succession.  I vaguely
suspect it has something to do with unix:.sockets/gdb, but I
actually have no idea.
gdb is considered busy when it's exited
Killing gdb is more deterministic

There's still a race condition on restarts, but I think it's an
issue in the startup code
minor code reorganization
moved to async.queue for gdb interaction queue

closes https://github.com/jaredp/OS161-Web-GDB/issues/27