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.