The MRF persister accumulated overflow entries in `pending`, flushed them with
`flush_mrf_to_disk`, and cleared `pending` on success. But `flush_mrf_to_disk`
*overwrites* the whole MRF file with exactly the entries passed. After flushing
batch A (file = A) and clearing, the next flush wrote batch B and thereby
overwrote the file to contain only B — and the MRF file is only replayed (and
cleared) at startup, never during the run, so batch A's entries were silently
lost. A crash after the B flush lost all of batch A's pending replications.
Keep `pending` cumulative (the file must hold the full set of overflow entries
for the run) and rewrite the whole set on each flush instead of clearing after
success:
- flush eagerly once 1 000 *new* entries accumulate since the last write
(measured against the flushed length, so a large backlog isn't rewritten on
every add), and on the 10s tick when dirty;
- bound the in-memory/on-disk backlog with `MRF_PENDING_CAP` (200 000) and log
once when the cap is hit rather than growing without limit.
Refs backlog#799 (B10).
