GIRAPH-1081: Fix a bug in internal out-of-core infra: multithreaded accesses to buffers

Summary: The multi-threaded accesses to raw data buffers in `DiskBackedDataStore` is overlooked, violating assumption on properly partitioning data to different IO threads.

Test Plan: mvn clean verify

Reviewers: dionysis.logothetis, sergey.edunov, maja.kabiljo

Reviewed By: maja.kabiljo

Differential Revision: https://reviews.facebook.net/D60147
2 files changed