ItemSync: Only fetch the items that are actually required.

ItemSync used to fetch *all* items during *every* sync, which is in most cases
completely unnecessary (at least for incremental sync). This patch delays the
fetch of items until we know which ones we need, and then only fetches the
required ones (usually none), resulting in an IMAP sync going from 10s 100%,
to a couple of seconds 1% cpu.

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