Implement batch database sample curator.

This commit introduces to Prometheus a batch database sample curator,
which corroborates the high watermarks for sample series against the
curation watermark table to see whether a curator of a given type
needs to be run.

The curator is an abstract executor, which runs various curation
strategies across the database.  It remarks the progress for each
type of curation processor that runs for a given sample series.

A curation procesor is responsible for effectuating the underlying
batch changes that are request.  In this commit, we introduce the
CompactionProcessor, which takes several bits of runtime metadata and
combine sparse sample entries in the database together to form larger
groups.  For instance, for a given series it would be possible to
have the curator effectuate the following grouping:

- Samples Older than Two Weeks: Grouped into Bunches of 10000
- Samples Older than One Week: Grouped into Bunches of 1000
- Samples Older than One Day: Grouped into Bunches of 100
- Samples Older than One Hour: Grouped into Bunches of 10

The benefits hereof of such a compaction are 1. a smaller search
space in the database keyspace, 2. better employment of compression
for repetious values, and 3. reduced seek times.
Merge pull request #169 from prometheus/feature/storage/compaction

Curator and Compaction Processor