Move the "activate linked connections" logic to a postloop event.

A linked connection_t is one that gets its I/O, not from the
network, but from another connection_t.  When such a connection has
something to write, we want the corresponding connection to run its
read callback ... but not immediately, to avoid infinite recursion
and/or event loop starvation.

Previously we handled this case by activating the read events
outside the event loop.  Now we use the "postloop event" logic.
This lets us simplify do_main_loop_once() a little.