Make manualPull send push requests over XMPP.
When reconnecting with remotes, those that are XMPP remotes cannot
immediately be pulled from and scanned, so instead maintain a set of
(probably) desynced remotes, and put XMPP remotes on it. (This set could be
used in other ways later, if we can detect we're out of sync with other
types of remotes.)
The merger handles detecting when a XMPP push is received from a desynced
remote, and triggers a scan then, if they have in fact diverged.
This has one known bug: A single XMPP remote can have multiple clients
behind it. When this happens, only the UUID of one client is recorded
as the UUID of the XMPP remote. Pushes from the other XMPP clients will not
trigger a scan. If the client whose UUID is expected responds to the push
request, it'll work, but when that client is offline, we're SOL.
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