blk: 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 wrong block sizes during opening of pools.
 - Add invariant checks inside btt where appropriate.
 - Remove unnecessary callback entry point

blk: pass rdonly flag to btt_init, no layout equals inconsistent

Ref: pmem/issues#5

These modifications are only valid when pmemblk_map_common is called
with the rdonly flag set - for examble in pmemblk_check.

The blk_nblock tests can no longer call pmemblk_check, because they
perform no writes to the pool. Therefore the layout is never written
and pmemblk_check returns an error each time.

The blk_rw tests have to be modified to check consistency after any
writes to the pool are performed.