imap: FETCH: proper MSN => UID mapping for requests
This finally seems to make mutt header caching behave properly.
We expect to be able to safely load 50K IV/UVs in memory without
OOM, since that's "only" 1.6 MB that won't live beyond a single
event loop iteration. So create a simple array which can
quickly map MSNs in requests to UIDs and not leave out messages.
MSNs in the FETCH response will NOT be correct, since it's
inefficient to implement properly and mutt doesn't seem to
care.
Since the conversion code is easily shared, "UID SEARCH" can
allow the same MSN => UID mapping non-UID "FETCH" does.