Use a shuffling algorithm to find unused IAX2 call numbers.

While adding red-black tree containers to astobj2 in r376575, Richard pointed
out the way chan_iax2 finds unused call numbers will prevent ao2_container
integrity checks at runtime.

This patch removes the ao2_container and instead uses fixed sized arrays and a
modified Fisher-Yates-Durstenfeld shuffle to maintain the call number list.

While the locking semantics are similar to the ao2_container implementation,
this implementation should be faster and more memory efficient.

Review: https://reviewboard.asterisk.org/r/2288/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@381427 f38db490-d61c-443f-a65b-d21fe96a405b