Close the connection.
When a connection has been closed, access to any of its methods
except this close() will cause an an IOException to be thrown.
Closing an already closed connection has no effect. Streams
derived from the connection may be open when method is called.
Any open streams will cause the connection to be held open
until they themselves are closed. In this latter case access
to the open streams is permitted, but access to the connection
is not.
