Meta: Make distributed deployments possible

In the light of #277 and Project Hexanet's internal needs. Here is the list of things to do:

- [x] #259

Refactoring:
- [x] #303
- [x] Replace updatepipe package written for go-imap v1 with a better update dispatcher.
- [ ] Implement support for some distributed broker - to be used with IMAP updates distribution.
- [ ] #262.
- [x] #304

Several queries should be optimized/reworked in go-imap-sql in order to behave well in a distributed environment:
- [ ] https://github.com/foxcpp/go-imap-sql/issues/28
- [ ] https://github.com/foxcpp/go-imap-sql/issues/33

Officially support CockroachDB:
- [x] https://github.com/foxcpp/go-imap-sql/issues/5
- [ ] Officially support CockroachDB use with auth.sql_table/auth.sql_query.

In other words, we have a long way to go. :)