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
add bump_credit to vq soak tests
merge default into bug25589
Merge bug25589