Undefined behavior from signed overflow in cursor manager, can result in mongos invariant failure

David Storch pointed this out while we were reading some code at my desk:

The call to std::abs(_pseudoRandom.nextInt32()) here could result in undefined behavior if _pseudoRandom.nextInt32() returns MIN_INT. Most compilers will probably do something reasonable in this situation. Many will probably just return MIN_INT, meaning that it's possible to get a negative cursor id.