tls: refactor write queues away

The TLS implementation previously had two separate queues for
WriteWrap instances, one for currently active and one for
finishing writes (i.e. where the encrypted output is being written
to the underlying socket).

However, the streams implementation in Node doesn’t allow for
more than one write to be active at a time; effectively,
the only possible states were that:

- No write was active.
- The first write queue had one item, the second one was empty.
- Only the second write queue had one item, the first one was empty.

To reduce overhead and implementation complexity, remove these
queues, and instead store a single `WriteWrap` pointer and
keep track of whether its write callback should be called
on the next invocation of `InvokeQueued()` or not
(which is what being in the second queue previously effected).

PR-URL: https://github.com/nodejs/node/pull/17883


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>