Configuration for Stasis threadpool

The appropriate settings for the Stasis threadpool is very system
specific, depending upon both workload and system configuration.

This patch adds a stasis.conf file which can be used to configure the
key attributes of the threadpool for the Stasis message bus.

(closes issue ASTERISK-21280)
Review: https://reviewboard.asterisk.org/r/2651/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393542 65c4cc65-6c06-0410-ace0-fbb531ad65f3