db_bench: set thread pool size according to max_background_flushes

Summary: option max_background_flushes doesn't make sense if thread pool size is not set accordingly. Set the thread pool size as what we do for max_background_compactions.

Test Plan: Run db_bench with max_background_flushes > 1

Reviewers: yhchiang, igor, rven, ljin

Reviewed By: ljin

Subscribers: MarkCallaghan, leveldb

Differential Revision: https://reviews.facebook.net/D24717
minor update to benchmark script

Summary: Try to match some parameters from Dhruba's benchmarks on github

Test Plan: ran it

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D24687
convert Options from string

Summary: Allow accepting Options as a string of key/value pairs

Test Plan: unit test

Reviewers: yhchiang, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D24597
Fixed compile error on Mac: default arguments for lambda expressions

Summary:
Fixed the following compile error on Mac.

db/db_test.cc:8618:52: error: C++11 forbids default arguments for lambda expressions [-Werror,-Wlambda-extensions]
  auto gen_l0_kb = [this](int start, int size, int stride = 1) {
                                                   ^        ~
1 error generated.

Test Plan:
db_test
Fixed Mac compile error in util/options_test.cc

Summary:
Fixed the following error in Mac:

./util/testharness.h:93:19: error: comparison of integers of different signs: 'const unsigned long' and 'const int' [-Werror,-Wsign-compare]
  BINARY_OP(IsEq, ==)
  ~~~~~~~~~~~~~~~~^~~
./util/testharness.h:86:14: note: expanded from macro 'BINARY_OP'
    if (! (x op y)) {                                   \
             ^
util/options_test.cc:269:3: note: in instantiation of function template specialization 'rocksdb::test::Tester::IsEq<unsigned long, int>' requested here
  ASSERT_EQ(new_cf_opt.write_buffer_size, 5);
  ^

Test Plan:
options_test
WriteBatchWithIndex to support an option to overwrite rows when operating the same key

Summary: With a new option, when accepting a new key, WriteBatchWithIndex will find an existing index of the same key, and replace the content of it.

Test Plan: Add a unit test case.

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24753
WriteBatchWithIndex's iterator to support SeekToFirst(), SeekToLast() and Prev()

Summary: Support SeekToFirst(), SeekToLast() and Prev() in WBWIIterator, returned by WriteBatchWithIndex::NewIterator().

Test Plan: Write unit test cases to cover the case.

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: rven, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D24765
WriteBatchWithIndex supports an iterator that merge its change with a base iterator.

Summary: Add an iterator that combines base_iterator of type Iterator* with delta iterator of type WBWIIterator*.

Test Plan: nothing yet. work in progress

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: rven, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D24741