Improve hashsocket to avoid collisions

[Coeur]

In 91e3ead, @nmathewson "improved" the hashsocket function with a "rotate-and-add trick".

Well, while rotating the bits is fine, performing an addition on the other hand will divide by two the number of possible different hashes. With twice less possible hashes, there will be collisions. Example:

unsigned h = 0x33333333;
h += (h >> 2) | (h << 30);
// result is 0xFFFFFFFF

unsigned h = 0xCCCCCCCC;
h += (h >> 2) | (h << 30);
// result is 0xFFFFFFFF

So we can keep the rotation, but we need to remove the addition for proper hashing.