ssl-params: slow startup (patch for consideration)

[Joseph Tam]

A. Schulze writes:

> precomputing ssl-params is also possible without patching but it's a
> little bit tricky
> ...
> Long version in german: https://andreasschulze.de/dovecot/ssl-params

Nice.

(You should probably point out to ensure ssl_parameters_regenerate is
zero, otherwise all this work will get wiped out!)

Joseph Tam <jtam.home at gmail.com>