Add is_open to check transport of connection

[sihyeonn]

To check whether the transport of connection is open or not, It would be great is_open method exists. Checking the connection first is helpful to prevent BrokenPIpeError from occuring and gives a chance to build a new connection.

AS-IS:

connection = hive.connect(host=_HOST)
print(connection._transport.isOpen())

TO-BE:

connection = hive.connect(host=_HOST)
print(connection.is_open())

[CLAassistant]

All committers have signed the CLA.

[sihyeonn]

@mdeshmu , this one is ready for CI and review. Thanks