Stasis performance improvements

Stasis performance improvements

This patch addresses several performance problems that were found in
the initial performance testing of Asterisk 12.

The Stasis dispatch object was allocated as an AO2 object, even though
it has a very confined lifecycle. This was replaced with a straight
ast_malloc().

The Stasis message router was spending an inordinate amount of time
searching hash tables. In this case, most of our routers had 6 or
fewer routes in them to begin with. This was replaced with an array
that's searched linearly for the route.

We more heavily rely on AO2 objects in Asterisk 12, and the memset()
in ao2_ref() actually became noticeable on the profile. This was
#ifdef'ed to only run when AO2_DEBUG was enabled.

After being misled by an erroneous comment in taskprocessor.c during
profiling, the wrong comment was removed.

Review: https://reviewboard.asterisk.org/r/2873/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@400138 65c4cc65-6c06-0410-ace0-fbb531ad65f3