Convert relay.c/relay.h to channel_t

Convert relay.c/relay.h to channel_t

Note: this is a squashed commit; see branch bug6465_rebased_v2 of user/andrea/tor.git for full history of the following 10 commits:

Convert relay.c/relay.h to channel_t
Updating the timestamp if n_flushed > 0 at the end of channel_flush_from_first_active_circuit() was redundant since channel_write_cell() et al. do it themselves.
Get rid of now-unnecessary time parameter in channel_flush_from_first_active_circuit()
Get rid of now-unnecessary time parameter in channel_flush_from_first_active_circuit() in connection_or.c
Add non-inlined external call for channeltls.c to free a packed_cell_t
Appease make check-spaces in relay.c
Replace channel_get_write_queue_len() with sufficient and easier to implement channel_has_queued_writes() in relay.c
Rename channel_touched_by_client() and client_used field for consistency with other timestamps in relay.c
Don't double-free packed cells in relay.c (channel_t Tor now bootstraps and works as a client)
Rearrange channel_t struct to use a union distinguishing listener from cell-bearing channels in relay.c