This allows to have better atomicity than shell redirection

TODO:
* support more TLDs

See merge request !12

Pack dns, http, network, ssl tables.

Vacuum the DB.

* reduce information density
* skip url not to check
* warn 2 weeks before ssl certificate expiration
* skip ssl/server warning if not a critical url

Allow to skip some top domains like .co.uk

This is low level timeout error, which can happend outside of socket.timeout,
see https://docs.python.org/3/library/exceptions.html#TimeoutError

/reviewed-on !8

The goal is to detect speed regression of website access.

The elapsed time value is updated in the DB only if an arbitrary threshold is modified (<200ms = fast, <500ms = moderate, >500ms = slow).

Cetificates are monitored until they expired.

Calling datetime.datetime.now will produce local time, whereas utcnow will
result with UTC time.

/reviewed-on nexedi/surykatka!2