[RocksDB] Reduce memory footprint of the blockbased table hash index.

Summary:
Currently, the in-memory hash index of blockbased table uses a precise hash map to track the prefix to block range mapping. In some use cases, especially when prefix itself is big, the memory overhead becomes a problem. This diff introduces a fixed hash bucket array that does not store the prefix and allows prefix collision, which is similar to the plaintable hash index, in order to reduce the memory consumption.
Just a quick draft, still testing and refining.

Test Plan: unit test and shadow testing

Reviewers: dhruba, kailiu, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19047
PlainTable to encode to avoid to rewrite prefix when it is the same as the previous key

Summary:
Add a encoding feature of PlainTable to encode PlainTable's keys to save some bytes for the same prefixes.
The data format is documented in table/plain_table_factory.h

Test Plan: Add unit test coverage in plain_table_db_test

Reviewers: yhchiang, igor, dhruba, ljin, haobo

Reviewed By: haobo

Subscribers: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D18735
Support for compaction filters in the C API
Add a way to set compaction filter in the C API
[RocksDB] Fix unit test

Summary: fix a bug in D19047, which caused  DBTest.RecoverDuringMemtableCompaction to fail.

Test Plan: unit test

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19155
Add a test for using compaction filters via the C API
bug fix: iteration over ColumnFamilySet needs to be under mutex

Summary: asan_crash_test is failing on segfault

Test Plan: running asan_crash_test

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19149
fix make shared_lib compilation error

Summary: s/class ParsedInternalKey/struct ParsedInternalKey

Test Plan: make shared_lib

Reviewers: igor, yhchiang, sdong, haobo

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19173
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
Use same sorting for all level 0 files

Summary:
We decided that one of the long term goals is to unify level and universal compaction.

As a small first step, I'm unifying level 0 sorting methods.

Previously, we used to sort level 0 files in level compaction by file number and in universal compaction by sequence number.

But it turns out that in level compaction, sorting by file number is exactly the same as sorting by sequence number.

Test Plan:
Ran make check with bunch of asserts to verify the sorting order is exactly the same.
Also, make check with this patch

Reviewers: haobo, yhchiang, ljin, dhruba, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19131
Remove seek compaction

Summary:
As discussed in our internal group, we don't get much use of seek compaction at the moment, while it's making code more complicated and slower in some cases.

This diff removes seek compaction and (hopefully) all code that was introduced to support seek compaction.

There is one test case that relied on didIO information. I'll try to find another way to implement it.

Test Plan: make check

Reviewers: sdong, haobo, yhchiang, ljin, dhruba

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19161
Fix compile
JSONDocument

Summary:
After evaluating options for JSON storage, I decided to implement our own. The reason is that we'll be able to optimize it better and we get to reduce unnecessary dependencies (which is what we'd get with folly).

I also plan to write a serializer/deserializer for JSONDocument with our own binary format similar to BSON. That way we'll store binary JSON format in RocksDB instead of the plain-text JSON. This means less storage and faster deserialization.

There are still some inefficiencies left here. I plan to optimize them after we develop a functioning DocumentDB. That way we can move and iterate faster.

Test Plan: added a unit test

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18831
Fix release compile
fix valgrind test
Merge master into windows.

Conflicts:
	db/db_impl.cc