blk: pad flog entries and deal with map entries in CL-sizes

Also replace spin locks with mutexes since userspace implementation performs better that way.
As I understand, the goal was to disallow concurrent threads to modify the map entries within the same cache line. I think the grouping introduced here is to coarse. The code presented here will select map_lock_nums from the set {0, 64, 128, 192} for nfree =256 ) the default value. Given that the map entries are uint32_t values, it would be better to set the lock alignment to 16. But I believe it would not be a good solution, because the grouping significantly limits the number of locks, thus adding to the contention.