Support for compaction filters in the C API
Add a way to set compaction filter in the C API
Add a test for using compaction filters via the C API
Merge pull request #179 from edsrzf/c-api-compaction-filter

Support for compaction filters in the C API
Fixed a potential write hang

Summary:
Currently, when something badly happen in the DB::Write() while the write-queue
contains more than one element, the current design seems to forget to clean up
the queue as well as wake-up all the writers, this potentially makes rocksdb
hang on writes.

Test Plan: make all check

Reviewers: sdong, ljin, igor, haobo

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19167
[RocksDB] Make block based table hash index more adaptive

Summary: Currently, RocksDB returns error if a db written with prefix hash index, is later opened without providing a prefix extractor. This is uncessarily harsh. Without a prefix extractor, we could always fallback to the normal binary index.

Test Plan: unit test, also manually veried LOG that fallback did occur.

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19191