Merge team/russell/iax2_performance.

Merge team/russell/iax2_performance.

There is not a large amount of code here and the changes are not very invasive.
However, they should significantly improve performance of chan_iax2 under load.

IAX2 media frames only carry the *source* call number.  So, when one arrives,
the correct session that it is a part of has to be matched on IP address, port
number, and call number, instead of just a call number.  Had these frames
carried the *destination* call number, this would not be an issue, because that
would be a unique identifier that would make it easy to immediately identify
the correct session.

The way that chan_iax2 did this matching was extremely inefficient.  It starts
at the first available call number and traverses each call number sequentially,
locking and unlocking a mutex for each one, to try to match against it.  It
would do this regardless of whether the call number was in use or not.  So,
for a call with a local call number of 25000, every single incoming media
frame would require a traversal that required 25000 mutex lock and unlock
operations.  (Note that the max call number is about 32k).

I have introduced a hash table of active IAX2 calls to improve this lookup
process.  The hash is done on the IP address, port number, and call number.
So, for the previous example, a few lock/unlock operations may be done versus
25000 for each frame.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@56447 65c4cc65-6c06-0410-ace0-fbb531ad65f3