chan_sip: Reorder unload_module to deal with stuck TCP threads.

chan_sip: Reorder unload_module to deal with stuck TCP threads.

In some situations TCP threads may become frozen.  This creates the
possibility that Asterisk could segfault if they become unfrozen after
chan_sip has been dlclose'd.  This reorders the unload_module process to
allow abort if threads do not exit within 5 seconds.

High level order as follows:
1) Unregister from the core to stop new requests.
2) Signal threads to stop
3) Clear config based tables (but do not free the table itself).
4) Verify that threads have shutdown, cancel unload if not.
5) Clean all remaining resources.

ASTERISK-26586

Change-Id: Ie23692041d838fbd35ece61868f4c640960ff882