Merge team/russell/ast_cli_tls into the trunk.

Merge team/russell/ast_cli_tls into the trunk.

This improves the performance of ast_cli() by not doing a heap memory
allocation and deallocation every single time the function is called. Instead,
a thread-specific buffer is allocatted the first time the function is called
and automatically free'd when the thread exits. Also note that this buffer will
only be allocatted in threads that actually call this function, which is
probably only the threads spawned to service connected asterisk consoles.

This does introduce a new limitation on the maximum length of the resulting
string from the arguments passed to ast_cli. Previously there was no limit
since it was just allocating a buffer big enough every time the function was
called. The current buffer size is 16kB. If there is ever a case where we want
to print more than 16k characters in a single call to ast_cli(), this will have
to be increased.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@38127 65c4cc65-6c06-0410-ace0-fbb531ad65f3