Enhance Python strValStore speed with generator

[weiliw-amz]

Issue #, if available:
N/A

Description of changes:
Further enhance Python String ValStore speed:

For 66M rows, 10 cols string store, batch get (threads=1) of sub-matrix time cost is reduced by 20%:

500 x 5: 2.5 ms -> 1.9 ms
100 x 5: 0.48 ms -> 0.39 ms

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.