borg2: compact always invalidates the whole index cache of clients

[ThomasWaldmann]

because compact always writes all index information into 1 new index file and deletes all the old index pieces, clientside caches become stale at that point and all clients need to refresh their cache and load that big index piece.

as borg compact is moving objects to new packs and also removing unused objects, it is maybe not trivial to improve this - collect ideas below.

[ThomasWaldmann]

Cache invalidation can be made less frequent by less frequently compacting, obviously (e.g. once a month or so), if free space permits. borg prune can be run more frequently, it does not change the index.

The issue of having to load one big index file when invalidation happens could be reduced by writing the new index into multiple files, so clients with slow/bad connections have less problems. The split into 2^N index files could be by masking the topmost N bits of the packid of index entries.