Throw CommandTimeoutExpired exception when query is cancelled

[bgrainger]

Command timeouts are now implemented by first attempting to cancel the query server-side. If this succeeds, a MySqlException will be thrown with the code QueryInterrupted.

This proposes that the error code be changed to CommandTimeoutExpired, so it's uniform for whether soft- or hard-cancellation happens for a command timeout, and QueryInterrupted is used only for MySqlCommand.Cancel. This reverts the decision in #455 (comment):

If the server-side cancellation is successful, the Execute/Read/etc. method will throw a MySqlException with an ErrorCode set to MySqlErrorCode.QueryInterrupted. This is a change from current behaviour, which throws a MySqlException with MySqlErrorCode.CommandTimeoutExpired.

This change will restore the behaviour to how it was prior to 1.1.0. For clients that need to distinguish server-side vs client-side command timeout, an InnerException could hold the QueryInterrupted error code.

[bgrainger]

Added in 1.3.0.