Increase Logstash Gearman worker queue length.

Logstash seems to perform better when it is not running out of buffer
space. Increase the buffer size to reduce contention and give the
gearman workers more room to work when things are queueing up. This
should increase throughput.

Change-Id: Ie4b5cc7a3ae4be517012c20559f59071b7b15dd5