Report message bytes in quorum queue stats

[dcorbacho]

rabbit_fifo keeps track of message bytes ready and unacknowledged and publishes them with the queue stats. It uses the new message_size_handler to calculate message size.

[#161505138]

Types of Changes

• Bugfix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

[dcorbacho]

@kjnilsson @michaelklishin Message size is recalculated every time the messages is unack or returned to the queue. Should we add it to the message properties we handle in rabbit_fifo? Any possible downside to this?