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