[JNI] Fixed a column family related compile error.

Summary:
Fixed the following column family related compile error.

./java/rocksjni/write_batch.cc:211:66: error: cannot initialize a parameter of type 'rocksdb::ColumnFamilyMemTables *' with an lvalue of type 'rocksdb::MemTable *'
  rocksdb::Status s = rocksdb::WriteBatchInternal::InsertInto(b, mem, &options);
                                                                   ^~~

Test Plan:
make jni
make jtest

Reviewers: igor, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17577
macros for perf_context

Summary: This will allow us to disable them completely for iOS or for better performance

Test Plan: will run make all check

Reviewers: igor, haobo, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17511
Small speedup of CompactionFilterV2

Summary: ToString() is expensive. Profiling shows that most compaction threads are stuck in jemalloc, allocating a new string. This will help out a litte.

Test Plan: make check

Reviewers: haobo, danguo

Reviewed By: danguo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17583
db_bench cleanup

Summary:
clean up the db_bench a little bit. also avoid allocating memory for key
in the loop

Test Plan:
I verified a run with filluniquerandom & readrandom. Iterator seek will be used lot
to measure performance. Will fix whatever comes up

Reviewers: haobo, igor, yhchiang

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17559
Remove env_ from MergingIterator

Summary: env_ is not used. Compiling for iOS complains.

Test Plan: compiles now

Reviewers: ljin, haobo, sdong, dhruba

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17589