Add support for playing back files recorded by slave backends

I finally managed to setup a slave backend (with a dummy tuner).
The problem why we could not play recordings was, that we mixed up the connections for the recording. We requested new blocks from the master, but wanted to receive them from the slave.

The implementation is currently much simpler as in MythFrontend's and requires that the master backend has access to the recordings made by the slave. We only try to stream it from the master backend.

Playing back Live TV (using the dummy tuner) works without stuttering for me. Maybe the recent mutex changes fixed this?

@electrobs: The branch (slave-backends) is compatible with frodo. Could you please give it a try? I guess for your setup it should be fine?
(and sorry for messing up the initial ticket #92 , please continue discussion in here)