Ycsb Go with Prometheus metrics

Original: https://github.com/pingcap/octopus/tree/master/ycsb

Difference from the original:

• Removed all the non tidb/sql benchmarks (tikv, raw, coprocessor)
• Removed Prometheus push functionality
• Read DB name from the connection DSN URL (and remove hardcoded DB name), also remove DB creation code
• Support custom SQL table for benchmark, configurable via CLI args