24594: ts: Initial TimeSeries TestModel. r=mrtracy a=mrtracy

Creates a new "testmodel" package under "ts" which contains an in-memory
test model designed to be much more conceptually simple than the
optimized query engine actually used by time series.

The new test model stores time series in sorted arrays as full-fidelity
nanosecond data points. It then uses a highly decomposed set of
functions to process that data; these functions have been designed to be
individually tested.

The "Query" method shows an example of how simple this system is when
compared to the actual time series system, which uses iterators and
breaks up queries into multiple chunks.

This model has not yet been hooked up to time series tests; it will soon
be used to replace the existing test model, which is *not* easier to
understand than the actual time series database.

Co-authored-by: Matt Tracy <matt@cockroachlabs.com>