Improve comments in packages codec and index.

Change-Id: Ic7b02ed97ea7ca3a69fc8e771f5e033e6658a8ec
Code cleanup in chunk.go and delta.go.

Also, implement GetBoundaryValue (not tested yet).

Change-Id: If1392d96f3d5b4cdde597b10c8dff1769fcfabe2
Merge branch 'exp/storage-ng' of github.com:prometheus/prometheus into exp/storage-ng

Conflicts:
	storage/local/index/interface.go

Change-Id: Iae7d3eac838f3d25ce0c88dd9543738b69acada8