Memory Pool optimizations

[eruffaldi]

1 - align to next pow2: useful for the little variable blocks of x264 compressors. Hint from user
2 - skip for smaller then size K: probably makes not necessary previous one
3 - per-task memory pool with lightweight deleter: the concept is that we want to reduce the cost of multithreaded allocation so we employ:
3.1 - per-task or per-activity pool (reasonable given the usage patterns)
3.2 - deletion can be lock-less

[eruffaldi]

Added k with 16384