Meta: Make distributed deployments possible

[foxcpp]

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

• Migrate to go-imap v2 #259

Refactoring:

• Refactor imapsql blob storage interface to use modules framework #303
• 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.
• Create abstraction for SMTP queue storage #262.
• S3 blob storage #304

Several queries should be optimized/reworked in go-imap-sql in order to behave well in a distributed environment:

• Get rid of "marking" logic in message removal go-imap-sql#28
• Expunge does two (!) full table scans go-imap-sql#33

Officially support CockroachDB:

• CockroachDB support go-imap-sql#5
• Officially support CockroachDB use with auth.sql_table/auth.sql_query.

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

[savchenko]

Is this relevant to #473 by any chance?..

[foxcpp]

Yes, it is directly related. Currently, maddy does but support any deployment setup using active replication. For MX redundancy, your best solution is to configure second MX to buffer incoming messages and attempt resending them to the main server infinitely.