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.
We need to document namespacing rules here. Certain values (here, "TIME_SERIES") are being treated as globally-reserved words by the merge operator. If this is to be usable by application code, we should have some rule for choosing non-colliding names.

Also, an 11-byte string (perhaps longer if we add namespacing) on every timeseries value seems like a lot. It might be worthwhile to follow the example of `integer` and use a different proto field for each merge operation we support.