Restrict the time series order in MTree

Currently, we maintain the order of time series by hash. It's better to give users a definite order, either in alphabet order or created order.

Start by locating the MTree implementation and reading how time-series order is currently maintained by hash. Clarify whether alphabetic or creation order is required, then define and test the chosen ordering behavior; the issue names no files or tests.

Written by the indexing model from the issue text.