Workaround for metadata request breaking other commands.

Summary:
Trojita sends this in a single packet:
y1 ID (\"name\" \"Test\")\r\ny2 ENABLE QRESYNC\r\ny3 LIST \"\" \"%\" RETURN (SUBSCRIBED))\r\n

After fixing the clientside in guam we now have the problem that the
replies for y1 and y2 get eaten by eimap waiting for the getmetadata
response, triggered by request y3.

A proper solution will require more invasive changes.
This seems to work with trojita for that specific sequence, for
everything else we'll have to see what we get in testing.

Differential Revision: https://git.kolab.org/D1141