…uffer size

This config value is largely ignored for now, but it is passed to places where
we'll need it in the next few commits.

The new functions can compute the maximum run size for a tiering/levelling run
given a level number, or conversely, can compute the level number of a
tiering/levelling run given the run's size. These alternative calculations are
parameterised by the maximum write buffer size, which is configured using the
`configMaxWriteBufferSize` field of `LSMConfig`.

As a sanity check for the run size calculations, some properties are added to
the prototypes test suite.

These alternative calculations are also closer to the real implementation of the
merge schedule.

And remove the old calculations.

And add a test, and immediately start using the alternative calculations in the
prototype.