saslserv/main: Track EID we're pending login to

The existing model does not remember that we've sent a SVSLOGIN for a
given SASL session, and simply assumes that if a client is introduced
with a SASL session open, that session must have succeeded. The security
of this approach requires ircd to implicitly abort SASL sessions on
client registration.

This also means that if a client successfully authenticates and then
does something else its pending login is forgotten about, even though a
SVSLOGIN has been sent for it, and the ircd is going to think it's
logged in.

This change removes the dependency on ircd's state machine by keeping
explicit track of the pending login, i.e. the one we've most recently
sent a SVSLOGIN for. The next commit will ensure that a client abort
(even an implicit one) doesn't blow that information away.