Update for MQ V9.3.1

• Update for MQ 9.3.1
• mqmetric - New metric channel_cur_inst. All instances of a given channel name have this aggregated value. Done this way because the other labels for a channel object (eg jobname) make them unique so harder to see how many exist with the same basic name
• mqmetric - Add metrics for AMQP channels
• mqmetric - Add cluster name as tag for queues (#191)
• Rewrite README to better match recent compiler versions and module management
• Update expected Go compiler version