Defer sending a KeyUpdate until after pending writes are complete

If we receive a KeyUpdate message (update requested) from the peer while
we are in the middle of a write, we should defer sending the responding
KeyUpdate message until after the current write is complete. We do this
by waiting to send the KeyUpdate until the next time we write and there is
no pending write data.

This does imply a subtle change in behaviour. Firstly the responding
KeyUpdate message won't be sent straight away as it is now. Secondly if
the peer sends multiple KeyUpdates without us doing any writing then we
will only send one response, as opposed to previously where we sent a
response for each KeyUpdate received.

Fixes #8677

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/8773)