tpcc: refresh conn if pingContext fails

[mahjonp]

Signed-off-by: mahjonp [email protected]

This PR fixes the invalid connection error when the tidb-server is killed

[mysql] 2021/03/27 07:50:25 packets.go:36: unexpected EOF
[2021-03-27 07:50:25] execute run failed, err invalid connection
[2021-03-27 07:50:25] execute run failed, err invalid connection
[2021-03-27 07:50:25] execute run failed, err invalid connection
execute run failed, err invalid connection
execute run failed, err invalid connection
execute run failed, err invalid connection
[2021-03-27 07:50:25] execute run failed, err invalid connection
execute run failed, err invalid connection
[mysql] 2021/03/27 07:50:25 packets.go:36: unexpected EOF
[mysql] 2021/03/27 07:50:25 packets.go:36: unexpected EOF

[cosven]

rest LGTM