Finish caching support for ScoreServer

[welchr]

Most of the code for caching score statistic segments has been implemented, but it needs to be finished/tested.

For now, the server seems to be "fast enough" for the small regions/numbers of genes we're handling. It may become more necessary however as testing on real data continues.

Relevant parts:

https://github.com/statgen/LDServer/blob/9a94a7d508be39c2d62659a5346a4b800fdb0c01/core/src/ScoreServer.cpp#L192

https://github.com/statgen/LDServer/blob/9a94a7d508be39c2d62659a5346a4b800fdb0c01/core/src/ScoreServer.cpp#L91-L102