Option to Disable Incoming mail during XML import

Related to JRA-28470 we want to complete the loop on this.

When restoring data, it's too easy to forget to add the jvm args to disable the incoming mail services.

If you are restoring a test / staging server, you can inadvertently end up pulling emails out of your inbox before the production JIRA has a chance to get to them. This causes a substantial amount of missing data from a production instance and can be difficult to track down (since it'll work for some emails, but not all, and it's purely based on which JIRA gets in first).

Essentially, just like the outgoing options, we need an option to disable incoming mail when restoring data.