Feature Request: automatically reconnect to TiDB

[kolbe]

The benchmark executor should automatically reconnect to TiDB and restart any aborted transactions if the connection to TiDB is broken.

High Availability is a core part of the TiDB value proposition, and any situation involving upgrading or failing over to another AZ/region will require applications to re-establish connections to TiDB, so our benchmarking tools should also support reconnections.

[zyguan]

Actually, it will automatically reconnect to TiDB if you run a workload with --ignore-error (except for TPCH and will be fixed by #147).