At the four-partial cap, retire the smallest partial batches only until
the new subjects fill a batch (at least one when they would leave a
partial of their own), instead of re-reading every partial.
Replace a batch whose subject was evicted and tracked again before the
sync, so its check mark and any pruned cached head are restored.
Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@localhost>
Signed-off-by: Tom Brow <tomb@squareup.com>
e2fb02ce0b