[RocksDB] Unit test to show Seek key comparison number
[RocksDB] Unit test to show Seek key comparison number

Summary: Added SeekKeyComparison to show the uer key comparison incurred by Seek.

Test Plan:
make perf_context_test
export LEVELDB_TESTS=DBTest.SeekKeyComparison
./perf_context_test --write_buffer_size=500000 --total_keys=10000
./perf_context_test --write_buffer_size=250000 --total_keys=10000

Reviewers: dhruba, xjin

Reviewed By: xjin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12843

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