Redesign space calculation algorithm for thin data/metadata
Redesign space calculation algorithm for thin data/metadata

The changes in this algorithm are as follows:
* Fix the convergence criteria to be the upper and lower bound are
  exactly 1 block apart
* Start the lower bound at 0 as 0 will always be small enough to fit on
  the device no matter what the filesystem limit is

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd