refined chokidar watch on new server additions

watcher only looks 1 depth--if beyond that, it is ignored.  this catches
any and all base_dir/servers/xxxx.
cleaned up addDir chokidar code
file tail handles contained in js object
watcher created if tailed file doesn't yet exist

example case:

server created
server tracked (but no logs/latest.log)
watch created for logs/latest.log)
server started
-logs/latest.log found, watcher closed
-tail for logs/latest.log started
server dispatcher created

dispatcher looks introspectively at function arguments and ensures all
required args are present in provided emission from client.

if true, execute the command, return callback