openssd: add some comments
openssd: insert ref counting for openssd_pool_block

We wish to keep track of outstanding I/Os issued to a specific
flash block. We do to avoid performing a on device garbage
collection of data, if we have outstanding reads on a to-be-gc'ed
pool block.
openssd: move defines and structs to header

We split the source into a core, gc, hint part for easier
maintainance of the code.
openssd: move gc related functions to own file

We want to implement various GC's, this will clutter the core file.