Limit memory consumed by dedup hash index

[tasket]

Wyng currently only curbs the growth of its (ephemeral) hash index when parts of the hash tree get full... Each tree branch references up to 65536 hashes. Even when that limit is reached on certain branches, others can continue to fill up.

Putting a limit on the index size in proportion to system RAM could reduce the risk of bogging down the system or overrunning /tmp. Some programs use a circular index (like circular buffer) to enforce limits, however Wyng has the advantage of chronologically arranged hash data in the manifests. The manifests could be read in reverse order when building the index and simply stop when the determined limit is reached.

The components of the index should be simple and deterministic enough to meter index growth with a cumulative variable instead of continually polling system memory stats.