The use of 'log' for all the large sizes was pretty confusing. Let's
use 'chunk' to describe the large alloc size. Other things live in them
as well as logs. Then use 'log segment' to describe the larger log
structure stored in a chunk that's made up of all the little blocks.
Get rid of the explicit distinction between brick and block numbers.
The format is now defined it terms of fixed 4k blocks. Logs become a
logical structure that's made up of a fixed number of blocks. The
allocator still manages large log sized regions.
