limit alpha->beta conversion through msg store credit flow
we also bump the IO_BATCH_SIZE; it's only used to limit beta->gamma
conversion, which is a fairly cheap operation (it only touches qi,
which has a very compact on-disk representation), and we want to make
sure that such conversion can keep up with the alpha->beta conversion.
merge in default
decouple syncing from paging
explain (reduced) impact of IO_BATCH_SIZE
propagate BQ API change
reduce_memory_use now only does one thing
increase IO_BATCH_SIZE
...so that beta->gamma conversion can keep up with alpha->beta
conversion.

Also correct comment - beta->gamma conversion can happen in a lot more
places than 'publish'
beta->delta conversion rate is naturally limited
which means we don't need an explicit upper bound, which in turn would
have required some interaction with q_process.
make tests work
- 'hole' generation is sensitive to IO_BATCH_SIZE
- need to handle credit bumps post publishing and prior to status
  checks
Show the distribution port as a "protocol" in management.
Add listeners to rabbitmqctl status.
Slightly simpler, and more polite.
Improved diagnostics.
Try to be a bit less verbose.
Since we always show the raw error and the expanded error let's get format_inet_error to do that.
Various formatting tweaks.
Just in case the error is not an atom.
Set the distribution port range to the first of these that matches:

* Not at all if it's mentioned in the config file
* $DIST_PORT if that's mentioned in the env file
* $RABBITMQ_DIST_PORT if that is set
* $RABBITMQ_NODE_PORT + 20000 if that is set
* 25672 in all other cases
Don't suggest using the kernel params here.
Slightly cleaner than iterating through families.