Postfix-mta-sts-resolver

This release splits main package dependencies into optional extras in order to reduce efforts of distro packages maintainers.

Changes:
* Installation procedure changed. Now full-featured package is pulled with `pip3 install postfix-mta-sts-resolver[sqlite,redis]` command
* Changed default config location to untangle it from postfix config directory. New location is `/etc/mta-sts-daemon.yml`

* Manifest application requests with User-Agent header

Switched to aiosqlite 0.10.0 which more precisely fits connection pooling demands

Fixes Python3.5 compatibility

* Fixes wait_closed() issue.
* Added thread pooling, timeouts and WAL for SQLite cache.
* Added cache grace feature: do not update and refresh records which are too fresh.

This release introduces sd_notify in daemon and allows finer reload control