SERVER-14413 Make dur::startup not recursively depend on globalStorageEngine
SERVER-15081 Improve DiskLoc::Hasher

The old implementation had a few problems. For one thing, it generated the
same hash for DiskLoc(1,2) and DiskLoc(2,1) as the combination operation
(xor) is commutative. Additionally the low order bits of the offset have low
entropy due to powerOfTwoSizing. Even without it, they are always at least
4-byte aligned.