Want non-memory-mapped persistence for StreamExistenceFilter

[timothycoleman]

While the stream existence filter is flushing (msync) there appears to be a small chance that ordinary writes can be delayed several seconds (highest observed was 25s on ZFS, GCP).

This may be ZFS specific, possibly related (though not identical) to openzfs/zfs#12662 for which reason we recently switched away from memory mapped checkpoints on zfs (#3332)

Plan is to create a non memory mapped version so that (1) we are no longer calling msync, avoiding the problem if it is ZFS related and (2) we take control of flushing directly so that we can control the rate and ensure it has a low impact on other disk operations regardless