driver/mysql: use go-sql-driver's multistatements

[fsouza]

I know that you've been looking into using mymysql instead of
go-sql-driver, but I don't think that this seems as a too big change. I
have some questions though:

1. is it fine to look for the string "multiStatements=true" in the
   driver url or should I parse the URL and ensure that it's defined to
   be true in the query string?
2. should we keep the fallback? I kept it because otherwise this change
   may break other people's flow/code
3. I changed migrate/migrate_test to test this change (included mysql
   to driverUrls, setting multiStatements=true). Should I commit this
   change as well?
4. Should I change or duplicate the test in mysql/mysql_test.go (I'd
   suggest testing both cases)

Any feedback is very appreciated.

The change is related to #1.

[mattes]

fixed in v3

migrate/database/mysql/mysql.go

Line 84 in be1ba92

purl.Query().Set("multiStatements", "true")