implement SessionResetter.

[shogo82148]

Description

Conn should implement driver.SessionResetter.

https://golang.org/pkg/database/sql/driver/#SessionResetter

database/sql will not return a bad connection to the connection pool.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

It's interesting idea.

I hope database/sql provides better way to close bad connection before return it to pool.
But this can mitigate current situation.

OTOH, it may reduce performance for "massive light query" pattern.
Would you try this benchmark, without compression?

[shogo82148]

Here is the result of the benchmark.

# master branch
$ go test -bench . -benchtime 60s
goos: darwin
goarch: amd64
pkg: github.com/go-sql-driver/mysql/bench
BenchmarkSmall-4   	  200000	    385242 ns/op	     482 B/op	      14 allocs/op
BenchmarkPlain-4   	   20000	   5732152 ns/op	  653555 B/op	   20413 allocs/op
PASS
ok  	github.com/go-sql-driver/mysql/bench	254.180s

# this branch
$ go test -bench . -benchtime 60s
goos: darwin
goarch: amd64
pkg: github.com/go-sql-driver/mysql/bench
BenchmarkSmall-4   	  200000	    384629 ns/op	     482 B/op	      14 allocs/op
BenchmarkPlain-4   	   20000	   5830082 ns/op	  653559 B/op	   20413 allocs/op
PASS
ok  	github.com/go-sql-driver/mysql/bench	255.172s

$ benchcmp master.txt pr779.txt
benchmark            old ns/op     new ns/op     delta
BenchmarkSmall-4     385242        384629        -0.16%
BenchmarkPlain-4     5732152       5830082       +1.71%

benchmark            old allocs     new allocs     delta
BenchmarkSmall-4     14             14             +0.00%
BenchmarkPlain-4     20413          20413          +0.00%

benchmark            old bytes     new bytes     delta
BenchmarkSmall-4     482           482           +0.00%
BenchmarkPlain-4     653555        653559        +0.00%

[methane]

Is this log helpful?
Are there some scenarios which mc is closed but log is not printed yet?

[shogo82148]

No, it isn't.
I'll remove it.