pmem: Extend pmemblk_check functionality.

 - Update the checksum handling to handle BE systems.
 - Improve the way the chk-sums are calculated.
 - Adjust the checksum unittest to be endianess aware.
 - Recover from failed flog map updates.
 - Handle worng blocksizes during opening of pools.
 - Add invariant checks inside btt where apriopriate.