Refactor oacd_freeswitch

[lordnull]

Primarily freeswitch_media, but some apply to all. The problems are:

• no unit tests: Using meck as a stand-in for the freeswitch module should make this easier.
• code duplication: freeswitch_media shares code with freeswitch_ring as well as freeswitch_dialer. Freeswitch_media_manager and freeswitch_monitor share some code. It seems like much of that can be abstracted out, to improve code re-use and ease future freeswitch developement.
• flawed apis: The freeswitch_media conference calls do not give useful return values. If a conference fails, the client is never informed. Those calls should be turned into calls that return a success or failure. This can be done using the freeswitch module, but requires changes to how agents communicate with freeswitch; the cpx_agent_connection module is mean to ease this type of communication.