Introducing a check inside BatchGetRequestBuilder.build() to fail when the key is CompoundKey or ComplexResourceKey. Adding support for BatchKVResponse to ScatterGatherBuilder as well as to BatchGetRequestBuilder.batch().

Modified Scatter Gather Tests to be consistent with the variations in the other Batch Get tests.

Added Changelog and incremented pegasus version.
Update release-version script to ignore untracked files.

RB=318542
R=gogenc,si-dev
A=gogenc