Use of inotify and possibly other mechanisms poses portability risks.
Notably, "cross-platform" abstractions such as fsnotify library remove
access to certain features that are important to use it correctly in
some cases e.g. it is preferable to listen only for IN_CLOSE_WRITE on
Linux instead of IN_MODIFY to prevent races and unexpected failures.
Pooling approach avoids such problems by either running reload code at a
different time than actual renewal or retrying later if parse fails.
With certificates being renewed before expiry (e.g. 1 week before) delay
is not a signficiant problem.
Closes#160.
DANE is complex and requires its own (potentially lengthy) tutorial on
how to configure it properly. MTA-STS, on the other hand, is designed to
be simple to deploy so recommend to configure it in the Setting up
tutorial.
