Simplify compaction and expose database sizes.

This commit simplifies the way that compactions across a database's
keyspace occur due to reading the LevelDB internals. Secondarily it
introduces the database size estimation mechanisms.

Include database health and help interfaces.

Add database statistics; remove status goroutines.

This commit kills the use of Go routines to expose status throughout
the web components of Prometheus. It also dumps raw LevelDB status
on a separate /databases endpoint.
Merge pull request #244 from prometheus/feature/database-statistics-exposition

Simplify compaction and expose database sizes.