Pull requests from #129 to ##203

Includes the following changes
- More strict HELO, EHLO, MAIL and RCPT command parsing
- Code clean-ups, style fixes, etc
- Fix EOF when parsing the envelope headers #142
- Fixes issue #148 - use the *validate* processor for validation
- FreeBSD compatibility
- SQL Statement configurable for different SQL databases, not just MySQL
- add able to configure DB connection limits in sql backend
- bug fix: server-log was incorrectly pointing to the main-log #180
- Support for TLS 1.3
- Issues #195 to #201 raised by @issuefiler, mostly fixing edge cases with handling IP addresse-literals and other inconsistencies related to rfc5321

Merge branch 'athoune-dep'

- update readme

- return error if can't bind