* Add rspamd_log variable to lua plugins to access logging functions

* Add rspamd_log variable to lua plugins to access logging functions
* Each part in rspamd task now can have parent part
* Check for parts distance only for multipart/alternative subparts
* Do not check attachements even if they are text (but attached as file)
* Do not die if write (2) returned ENOSPACE while doing logging, turn on throttling mode instead (1 write try in a second)
* Add ability to turn on debug for specific symbols
* Add ability to configure dns timeouts and dns retransmits in config file