read_timeout and write_timeout configuration settings.

[chakrit]

When the server that's running MySQL ran out of disk space, the mysqld process will hold onto all connections while it waits for disk space to be freed. Unfortunately there is currently no way to timeout the connection in this situation with go-sql-driver/mysql.

The existing timeout parameter is only used during initial connection establishment (driver.go:65).This PR adds read_timeout and write_timeout settings to the DSN and enforce them using net.Conn.SetReadDeadline and net.Conn.SetWriteDeadline respectively.

Let me know what you guys think.

• Code compiles correctly
• Created tests, if possible
• All tests pass

[methane]

LGTM.

I prefer adding readTimeout and writeTimeout to buffer instead of adding cfg. But it's my personal taste.

[julienschmidt]

Continued in #401