Channeltls adds the connection write event to main loop when writing a packed cell

This is another problem in the line of channel subsystem and KIST. It is a difficult problem because tor is wired to use libevent connection's write and read event where KIST needs to control what is put on the network at what time.

So, here is the callstack that leads to the channeltls layer adding the conn->write_event to the main loop:

channel_tls_write_packed_cell_method()
 -> connection_buf_add()
   -> connection_write_to_buf_commit()
     -> connection_start_writing()
       -> event_add(conn->write_event, NULL)

Basically, when we flush cell(s) from a circuit, we do call back into the channel subsystem to write the packed cell (only packed cell on a circuit queue). The libevent write event is then added to the main loop. Then KIST scheduler kicks in a tick later (10 ms) and can also try to write.

I don't see a direct impact here but we could avoid an entire "write_event" callback on a connection with the KIST scheduler which seems to me like a good optimization especially on fast relays.