Add MOVE support

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Yeah, there is an IMAP MOVE command. It is similar to the COPY, STORE /Deleted, EXPUNGE trio but in a single command. The semantics are the same as doing the above three commands except:

1. We never add the /Deleted flag to a message. They are just EXPUNGE'd as if they were in the Deleted sequence
2. need to guarantee folders remain in a consistent state throughout the process.

Conflicting commands: Since we do an implicit expunge, we conflict with non-UID store, search, fetch

(Do we need to re-think our conflict check? Mainly a lot of UID based messages do not conflict?)

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RFC6851