Bisectable should first try stuff that is not in long term storage

See #23 and #117 for some background.

Basically, all new commits (>2015.12) and all tagged commits are compressed with zstd (very fast decompression, but relatively low compression ratio), but old non-tagged commits are put into long term storage mode and are compressed with lrz (slow-ish decompression, but extremely high ratio).

When you ask bisectable to bisect something before 2015.12 (e.g. old=2014.01 new=2015.12), it will rarely stumble upon nice zstd archives. We should teach it to try these first (to speed things up), and that will allow us to use more aggressive compression strategy (for example: all builds up to HEAD in long term storage mode, but with a bunch of zstd-ed checkpoints every 5 commits)

Currently the difference is about 10 seconds vs 30 seconds. I was thinking that it's not a big deal if you really want to bisect that far, and it isn't. But as time goes on (and as rakudo development ramps up), I'm starting to run out of space again (I have a very small SSD on the server).

Start by tracing bisectable's archive-selection logic for ranges before 2015.12, using the background in #23 and #117. Done means bisection tries available zstd archives before long-term lrz archives while retaining the stated compression and storage behavior.