Commit 1811193285 caused Racket CS to have much higher peak memory
use. Adjust the heuristic again to trigger a major GC when the
`current-memory-bytes` value is the post-GC `bytes-allocated` plus the
post-GC `current-memory-bytes`, which means waiting until
another `bytes-allocated` bytes are allocated (instead of waiting
until the number of newly allocated bytes also catches up to overhead,
such as unserused pages due to locked objects).
1f2342b57a