Mergeable Time Series Data

This commit defines a new "TimeSeriesData" message which is used to record time
series data in an efficient way.  It also defines a custom "merge" operation for
TimeSeriesData within our existing RocksDB MergeOperator.

Background:
A "Time Series" is a set of data samples taken from the same statistic at
different points in time - for example, a common use of Time Series is the
recording of metrics for an application. Because of specialized query
requirements and necessarily high write performance, many specialized Database
solutions have been created specifically to handle Time Series data.

In Cockroach, multiple samples of a single statistic will be stored at a single
key; a key will contain all samples taken from a metric during over a well
defined time period such as a wall-clock hour. Based on our research into other
TS database solutions, this grouping is very effective in improving read
performance.

High write performance is enabled by creating a custom Merge operation for Time
Series data.  Whenever metric data samples are written, a Merge operation is
utilized instead of a put; for example, a single metric source will merge
samples into the same key at several points over the course of an hour.  The
resulting "merged" value will appear as if all samples had been written together
as a single value.
It's globally reserved by the Merge operator, but I'm not sure we're going to expose Merging to end users directly.  In that case, a user-created value would never be encountered by the merge operator.

I originally had a different proto field on Value which contained a TimeSeriesData, exactly as you describe. However, the Value message IS exposed directly to end users, and it seemed like this design would create a lot of confusion for end users.