Decide what to name the last request bucket

[gunnihinn]

In pull request 348 we had a discussion on whether to name the last millisecond bucket we keep around like the others or to name it Xms_to_inf. Both options are reasonable and we don't feel we've made a final call on the matter. It'd be nice to settle this either way, or add an option so users can pick which one they want.

[nnuss]

(also here) The history around why carbonzipper went with the naming scheme it did: https://github.com/go-graphite/carbonzipper/pull/18