Initial in-memory arena implementation.

It is unbounded, and nothing uses it except for a gating flag in main.
Add repetitious value benchmark.

The repetitious value benchmark will enable us to validate that the
new storage behaviors are both performant and correct.
Wrap in-memory arena values in interface.

This will segway into supporting repeated values efficiently.
Delete versions; SampleValue -> SampleValueSeries.

Versioning the data model is premature.  We get nothing from it now.

Given that SampleValue will be replaced with a repeating structure,
we should start the rename gracefully.
Incremental movement toward repeated samples.

This commit merely introduces repeated fields for sample values, but
nothing takes advantage of the compaction behaviors.
Started documenting new query semantics.