ds: stop distinguishing event read and write callbacks

ds: stop distinguishing event read and write callbacks

Having separate read/write callbacks in every class is too
confusing to my easily-confused mind.  Instead, give every class
an "event_step" callback which is easier to wrap my head around.

This will make future code to support IO::Socket::SSL-wrapped
sockets easier-to-digest, since SSL_write() can require waiting
on POLLIN events, and SSL_read() can require waiting on POLLOUT
events.