fix connection threads tracking leak

better mutex name, loop with while true

event per connection & thread cleanups specs

remove leftover

closes #1509
logstash boot sequence cleanup & drip support fix
deprecated USE_JRUBY environment var
added USE_RUBY and USE_DRIP environment vars
centralized gem path handling in LogStash::Environment

use consistent discovery strategy
Merge git://github.com/logstash/logstash