Use a MemBlock pod struct instead of std::pair in MemoryManager

Summary: I considered a typedef, or using folly::Range, or Slice, but
went with this in the end. Might change it to Range or Slice
later.

Reviewed By: @jdelong

Differential Revision: D1690997

Signature: t1:1690997:1416420124:787e811fea7bb68128545ae697ca47062c0b906d