mysql: replace mymysql with go-sql-driver

from github.com/go-sql-driver/mysql at 9a7aa3606b82e2081a13a008ada88dfdb96c20fd

Context: http://camlistore.org/issue/428

Change-Id: Ia93ac35da02933bd03d2c6b7b8e5e549ba0472cb
wip: race in camtool