Allow to disable the "overwriting not closed connection" warning

[amochin]

Currently, the library throws a warning, if the Connect to database keyword was called without closing the connection via the Disconnect from database keyword. This might be sometimes unwanted - e.g. when the connection has to be opened before each request and the overwriting is accepted.

One of the cases is when pymysql drops a connection after some timeout and throws an InterfaceError: (0, '') - in this case a new connection is needed each time and running a pair of "disconnect" + "connect" doesn't seem to be the most graceful solution.