Fix CVE-2018-12543. If a message is sent to Mosquitto with a topic that begins with $, but is not $SYS, then an assert that should be unreachable is triggered and Mosquitto will exit.
- Elevate log level to warning for situation when socket limit is hit.
- Remove requirement to use `user root` in snap package config files.
- Fix retained messages not sent by bridges on outgoing topics at the first connection. Closes #701.
- Documentation fixes. Closes #520, #600.
- Fix duplicate clients being added to by_id hash before the old client was removed. Closes #645.
- Fix Windows version not starting if include_dir did not contain any files. Closes #566.
- Various fixes to ease building.