Simplify supervisor tree around peers.

Just hook everything on the control process and link everyone to each
other. This way, a crash in a sender or receiver is now propagated as
a crash in the control process which are marked temporary. It quells
some warning messages that came up all the time.
Monitor pids

This should fix and close down stale workers.
Merge branch 'gh-7-2' into next

Conflicts:
	src/etorrent_peer_control.erl