LearnReadOrientationModel loads tables for one ref context at a time, reducing memory demands 32x

[davidbenjamin]

Closes #7948

[gatk-bot]

Github actions tests reported job failures from actions build 7405843720 Failures in the following jobs:

Test Type	JDK	Job ID	Logs
unit	17.0.6+10	7405843720.12	logs
integration	17.0.6+10	7405843720.11	logs
unit	17.0.6+10	7405843720.1	logs
integration	17.0.6+10	7405843720.0	logs