Split crystal_thresh into crystal_thresh + fragment_thresh

So now crystal_thresh only controls when fragments are compacted into
blocks, while fragment_thresh controls when blocks are broken into
fragments. Setting fragment_thresh=-1 will follow crystal_thresh and
keeps the previous behavior.

These were already two separate pieces of logic, so it makes sense to
provide two separate knobs for tuning.

Setting fragment_thresh lower than crystal_thresh has some potential to
reduce hysteresis in cases where random writes push blocks close to
crystal_thresh. It will be interesting to explore this more when
benchmarking.

---

The additional config option adds a bit of code/ctx, but hopefully that
will go away in the future config rework:

           code          stack          ctx
  before: 35584           2480          636
  after:  35600 (+0.0%)   2480 (+0.0%)  640 (+0.6%)

tests/test_fwrite.toml

@@ -5,8 +5,11 @@ after = 'test_files'
 # test with different fragment sizes
 defines.FRAGMENT_SIZE = [1, 16, 64]
 
-# test with different crystal sizes
-defines.CRYSTAL_SIZE = [512]
+# test with different crystallization thresholds
+defines.CRYSTAL_THRESH = [512]
+
+# test with different fragment thresholds
+defines.FRAGMENT_THRESH = [-1]
 
 # test with different prog sizes
 defines.PROG_SIZE = [1, 16]