Garbage collection of mirrored queue metrics on nodes that did not
host a master or mirror for a queue affected delivery and acknowledgement rates.
This could result in rates being 0 or negative when they should not be.
GitHub issue: rabbitmq-server#1340
Stats emission could prevent queue mirrors from performing garbage collection
and consume memory even when they were empty.
RABBITMQ_DISTRIBUTION_BUFFER_SIZE now can be set via
GitHub issue: rabbitmq-server#1338