Address JSON thread safety issues.

In tracking down some unit tests failures, I ended up reading the fine
print[1] regarding Jansson's thread safety.

In short:
 1. Ref-counting is non-atomic.
 2. json_dumps() and friends are not thread safe.

This patch adds locking where necessary to our ast_json_* wrapper API,
with documentation in json.h describing the thread safety limitations of
the API.

 [1]: http://www.digip.org/jansson/doc/2.4/portability.html#thread-safety

Review: https://reviewboard.asterisk.org/r/2716/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396119 65c4cc65-6c06-0410-ace0-fbb531ad65f3