Add separate Read/WriteUnlock methods in MutexRW.

Some platforms, particularly Windows, do not have a single method that can
release both a held reader lock and a held writer lock; instead, a
separate method (ReleaseSRWLockShared or ReleaseSRWLockExclusive) must be
called in each case.

This may also be necessary to back MutexRW with a shared_mutex in C++14;
the current language proposal includes both an unlock() and a
shared_unlock() method.
Merge pull request #176 from bgrainger/mutexrw-unlock

Add separate Read/WriteUnlock methods in MutexRW.
Add a table factory that can read DB with both of PlainTable and BlockBasedTable in it

Summary: The new table factory is used if users want to convert a DB from one table format to the other. A user can use this table to open a DB written using one table format and write new files to another table format.

Test Plan: add a unit test

Reviewers: haobo, igor

Reviewed By: igor

Subscribers: dhruba, ljin, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D19017
specify the command to install build_tools/mac-install-gflags.sh file in doc
Merge pull request #177 from nanwu/master

specify the command to install build_tools/mac-install-gflags.sh file in...
HashLinkList to log distribution of number of entries aross buckets

Summary: Add two parameters of hash linked list to log distribution of number of entries across all buckets, and a sample row when there are too many entries in one single bucket.

Test Plan: Turn it on in plain_table_db_test and see the logs.

Reviewers: haobo, ljin

Reviewed By: ljin

Subscribers: leveldb, nkg-, dhruba, yhchiang

Differential Revision: https://reviews.facebook.net/D19095
Change order of parameters in adaptive table factory

Summary:
This is minor, but if we put the writing talbe factory as the third parameter, when we add a new table format, we'll have a situation:
1) block based factory
2) plain table factory
3) output factory
4) new format factory

I think it makes more sense to have output as the first parameter.

Also, fixed a NewAdaptiveTableFactory() call in unit test

Test Plan: unit test

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19119