Alternate Channel Storage Backends

Alternate Channel Storage Backends

Full details: http://s.asterisk.net/dc679ec3

The previous proof-of-concept showed that the cpp_map_name_id alternate
storage backed performed better than all the others so this final PR
adds only that option.  You still need to enable it in menuselect under
the "Alternate Channel Storage Backends" category.

To select which one is used at runtime, set the "channel_storage_backend"
option in asterisk.conf to one of the values described in
asterisk.conf.sample.  The default remains "ao2_legacy".

UpgradeNote: With this release, you can now select an alternate channel
storage backend based on C++ Maps.  Using the new backend may increase
performance and reduce the chances of deadlocks on heavily loaded systems.
For more information, see http://s.asterisk.net/dc679ec3