blk: finish the implementation

- 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

Initial version by @mdalecki
Merge pull request #60 from tomaszkapela/pmemblk_finish_impl

blk: finish the implementation
pmem: split pmem_pool_open into pmem_pool_create and pmem_pool_open
Merge pull request #62 from krzycz/pmem-pool_create

pmem: split pmem_pool_open into pmem_pool_create and pmem_pool_open