ReadsContext class needs caching

[droazen]

Now that we have important VariantWalker tools that use reads as a side input (such as @lucidtronix 's CNNScoreVariants), we need to add caching to ReadsContext for good performance on nearby reads queries during a traversal.

The caching should be modeled after the existing caching in FeatureContext as implemented in the FeatureCache class, but it should include the ability to cache "around" the current locus, rather than just ahead of it as in FeatureCache. Ideally, the tool itself should be able to configure the default caching behavior via arguments to control bases to cache before and after current locus.

[droazen]

For @cmnbroad (who has already started on the implementation, I believe)

[droazen]

In review here: #4902