tls: expose SSL_CTX_set_timeout via tls.createServer

Add the `sessionTimeout` integral value to the list of options
recognized by `tls.createServer`.

This option will be useful for applications which need frequently
establish short-lived TLS connections to the same endpoint. The TLS
tickets RFC is an ideal option to reduce the socket setup overhead
for such scenarios, but the default ticket timeout value (5
minutes) is too low to be useful.
buffer: remove _charsWritten

_charsWritten is an internal property that was constantly written to,
but never read from. So it has been removed.

Removed documentation reference as well.