Summary:
This patch allows db_bench to initialize it's RocksDB Options via a
options file, specified by the --options_file flag. Note that if
--options_file flag is set, then it has higher priority than the
command-line argument.
Test Plan: db_bench_tool_test
Reviewers: sdong, IslamAbdelRahman, kradhakrishnan, yiwu, andrewkr
Reviewed By: andrewkr
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D58533
