Summary:
MyRocks hit a regression, @mung generated perf reports showing that the reason is the cost of calling `GetDBOptions()` inside `GetFromBatchAndDB()`
This diff avoid calling `GetDBOptions` and use the `ImmutableDBOptions` instead

Test Plan: make check -j64

Reviewers: sdong, yiwu

Reviewed By: yiwu

Subscribers: andrewkr, dhruba, mung

Differential Revision: https://reviews.facebook.net/D65151