support batch for mysql

[aylei]

This PR add batch support for mysql, which was motivated by using batch insert to accelerate the load process during benchmarks.

This commit was tested in benchmarks with 3 billions rows. Also, I did some smoke tests locally:

> tiup playground

> ./bin/go-ycsb load mysql -P workloads/workloada -p mysql.host=127.0.0.1 -p mysql.port=4000
***************** properties *****************
"readproportion"="0.5"
"batch.size"="50"
"recordcount"="100000"
"dotransactions"="false"
"scanproportion"="0"
"workload"="core"
"operationcount"="100000"
"mysql.host"="127.0.0.1"
"updateproportion"="0.5"
"insertproportion"="0"
"requestdistribution"="uniform"
"readallfields"="true"
"mysql.port"="4000"
**********************************************
BATCH_INSERT - Takes(s): 9.8, Count: 1807, OPS: 183.7, Avg(us): 4842, Min(us): 4011, Max(us): 63910, 99th(us): 8000, 99.9th(us): 33000, 99.99th(us): 57000
Run finished, takes 17.490709788s
BATCH_INSERT - Takes(s): 17.3, Count: 2000, OPS: 115.4, Avg(us): 4827, Min(us): 4011, Max(us): 63910, 99th(us): 8000, 99.9th(us): 33000, 99.99th(us): 64000

> ./bin/go-ycsb run mysql -P workloads/workloada -p mysql.host=127.0.0.1 -p mysql.port=4000
***************** properties *****************
"requestdistribution"="uniform"
"workload"="core"
"scanproportion"="0"
"updateproportion"="0.5"
"dotransactions"="true"
"operationcount"="100000"
"readallfields"="true"
"recordcount"="100000"
"batch.size"="50"
"insertproportion"="0"
"mysql.port"="4000"
"readproportion"="0.5"
"mysql.host"="127.0.0.1"
**********************************************
BATCH_READ - Takes(s): 9.9, Count: 215, OPS: 21.7, Avg(us): 1709, Min(us): 1404, Max(us): 3159, 99th(us): 3000, 99.9th(us): 4000, 99.99th(us): 4000
BATCH_UPDATE - Takes(s): 9.9, Count: 148, OPS: 14.9, Avg(us): 64719, Min(us): 60583, Max(us): 117298, 99th(us): 116000, 99.9th(us): 118000, 99.99th(us): 118000
........

Signed-off-by: Aylei rayingecho@gmail.com

[dbsid]

what's the default value of batch.size? can we keep the default behaviors consistent with previous version?

[aylei]

what's the default value of batch.size?

No batch by default.

can we keep the default behaviors consistent with previous version?

Yes, the default behavior would be consistent

[dbsid]

/lgtm