Add red-black tree container type to astobj2.

* Add red-black tree container type.

* Add CLI command "astobj2 container dump <name>"

* Added ao2_container_dump() so the container could be dumped by other
modules for debugging purposes.

* Changed ao2_container_stats() so it can be used by other modules like
ao2_container_check() for debugging purposes.

* Updated the unit tests to check red-black tree containers.

(closes issue ASTERISK-19970)
Reported by: rmudgett
Tested by: rmudgett

Review: https://reviewboard.asterisk.org/r/2110/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376575 65c4cc65-6c06-0410-ace0-fbb531ad65f3