561 emulate hangup for channel stuck at the end of hangup handlers

Closes #561

In asterisk we use hangup_handlers (known to be troubled).

In some attended transfer scenarios channel get stuck at the laste step (Return) of hangup_handlers. By emulating hangup event and deleting channel from ChannelRepository to ignore iurther events on this channel we compensate this asterisk's issue.

Note: by looking at output of asterisk -rx 'core show channels verbose' we thought, that call can get live again (if not picked -up by transferee). But in fact channel id we receive in xuc has one more character at the end discriminating new channel. So just to emit hangup & delete channel from ChannelRepository (to ignore further events on this channel esp. delayed hangup) is enough, no need to "resurrect" channel.