IOError cleanup

Summary: Clean up IOErrors so that it only indicates errors talking to device.

Test Plan: make all check

Reviewers: igor, haobo, dhruba, emayanke

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15831
CompactionContext to include is_manual_compaction

Summary: Added a bit more information to compaction context, requested by internal team at FB.

Test Plan: Modified CompactionFilter test to make sure is_manual_compaction is properly set.

Reviewers: haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16095
Support prefix seek in UserCollectedProperties

Summary: We'll need the prefix seek support for property aggregation.

Test Plan: make all check

Reviewers: haobo, sdong, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15963