openssd: alloc/dealloc of pools, block and aps.
openssd: remove dm_ naming

Writing dm_ in front of every structure get tiresome. Let's spice
it up, and remove it. Keep it for init and exit for dm. As it
make sense for these calls.
opensdd: handling of free/used blocks within a pool

Add helpers to synchronize access of blocks within a pool. We let
the block know about its parent pool, so that there is no chance
of the developer pushing a block to the wrong pool by a mistake.
openssd: round robin logical to physical map

We implement a logical to physical translation mapping using a
simple round-robin approach for append points.

There is still a need for gc, so the implementation fails when no
more free blocks are left.