Replace index writes with wrapped interface.

This commit is the first of several and should not be regarded as the
desired end state for these cleanups.  What this one does it, however,
is wrap the query index writing behind an interface type that can be
injected into the storage stack and have its lifecycle managed
separately as needed.  It also would mean we can swap out underlying
implementations to support remote indexing, buffering, no-op indexing
very easily.

In the future, most of the individual index interface members in the
tiered storage will go away in favor of agents that can query and
resolve what they need from the datastore without the user knowing
how and why they work.
Code Review: Nits.
Merge pull request #351 from prometheus/refactor/storage/componentize-index

Replace index writes with wrapped interface.