Add support for drip.

Drip allows for faster startup time by pre-starting JVMs and keeping a
pool of them around for use on-demand. More details here:
https://github.com/flatland/drip

It is *very* important to note that this only improves startup time, not run-time. It is recommended *ONLY* for testing/development iteration - not production.

Tested with this config:

* 'input { generator { count => 1 } } output { null { } }'

Without drip:

    % time bin/logstash -e '...'
    13.19s user 2.94s system 170% cpu 9.458 total

With drip (install drip, then set 'JAVACMD=drip' in env):

    % time JAVACMD=drip bin/logstash -e '...'
    0.03s user 0.46s system 6% cpu 7.008 total

2.5 seconds faster (27% faster). the nearly-zero user/system times are because 'time' only measures time spent in the 'drip' process, which at this point is simply a client telling a drip pool instance to execute.