Change the message store architecture from an index-based to a uid-based
one. Key advantage of this design approach is that no reselect mechanism
is required anymore since it comes with the design for free.
Fixes: https://todo.sr.ht/~rjarry/aerc/43
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>