Auto segmentation/paging for segment caching

[thorwhalen]

Define segmentation granularity (namely, of time) and store/retrieve under that granularity (that is, taking the smallest containing segments to respond to a query).

Examples;

• assume granularity is 100. Then when [423:746] is requested, the actual retrieval path is store[400:800][23:346]
• assume granularity is a month. Then when [2018-02-05:2018-07-09] is requested, [2018-02-01:2018-07-31] is retrieved but [2018-02-05:2018-07-09] is still returned.

This setup helps mitigating fragmentation when caching segments.