Complete rework of bufferevent event masking allowing for more

strictness:
- Drop clients attempting command pipelining; protects the daemon
from all kinds of abuse.
- Replace F_EVLOCKED flag with F_WRITEONLY which has cleaner sematics:
when up, session must not be destroyed nor read from, but may be
written to.
- Write callback becomes a central place for enabling EV_READ.
- Delay bufferevent creation until after ssl handshake is completed.

A bunch of session error stats were added to smtpctl's "show stats".
These could help spotting event masking errors in the future.

ok gilles@

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