borg2: compact: deal with tiny packs

[ThomasWaldmann]

if one runs borg create N times (with little changes in between, on a fresh repo):

• first run will typically create a lot of big/full packs (according to PackWriter settings) and maybe 1 final not-full pack with the remaining chunks
• 2nd+ runs will create only 1 tiny pack per run (most stuff is deduplicated, but the archive metadata chunk is different in any case, maybe also a few other chunks)

Current compaction policy is purely based on not-used size threshold.

The tiny packs are 100% used as long as the archives are not deleted, so they will be never (or rarely) touched by current borg compact code.

Guess there should not only be a max_size / max_count for packs, but also a min_size and min_count and borg compact should additionally create bigger packs from tiny packs.

[ThomasWaldmann]

Note: this can also call Store.defrag - the list of source blocks has (pack_name, offset, size) tuples, so it can collect blocks from multiple packs and create one new pack from them.