Enhancements for ComputeProperStatByWindow

[magicDGS]

• Create abstract engine class to support other enhancements, and construct with a builder
• Support unsorted files - the engine in this case should hold all the windows without flushing until the traversal is finished. This have a performance cost, but it will be useful to analyze raw mapped data before sorting.
• Add support for sliding-window analyses - I believe that the current implementation supports this traversal, but depends on how the engine is optimized (Evaluate performance improvements for ProperStatWindowEngine #458) if this will continue working.
• Remove no-interval parameters requirement. This depends on GATK making accessible the intervals for traversal (to make the windows) and GATK making a public method to change intervals for traversal within reads (before starting traversal). Another option is to implement a pair-end walker using something like TwoPassReadWalker: allow different ways for traversal in each pass broadinstitute/gatk#4849