extindex: fix boost with partial runs
Boost relies on knowledge of all inboxes in a given config file
to work properly. So while we support indexing a subset of
inboxes, we must still account for boost in inboxes we're not
indexing. So split internal inbox groups into "known" and
"active", where previously we only cared for inboxes which were
being actively indexed.
Furthermore, boost checks need to be applied when a
message arrives in different inboxes across multiple
invocations.
Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://public-inbox.org/meta/20210802204058.vscbxs5q7xyolyu2@nitro.local/