Unsupported URI options do not warn

The connection string spec requires that unsupported option keys result in a warning: A WARN level logging message MUST be issued

The Go driver connection string package accumulates UnknownOptions, but doesn't appear to do anything with them.

(This raises a secondary question of whether Go driver logging should use the log package or whether the Go driver should allow a logger to be injected into the client via options and used for logging.)