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
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