Previously a new call id was generated for INVITE and CANCEL.
Now the id generated during the initial INVITE will be used for
corresponding CANCEL events. Also, adding the ability to
trigger a call cancel via the poltergeist update api.

This should allow us to have a consistent convention and assist
with client translation of status strings.

When combined with mod_muc_poltergeist mod_muc_call allows
for enabling call features using a proper ext_events.lib.lua
implementation. By default when the module is configured only
stub implementations are used for ext_events.lib.lua as these
are unique between deployments.