fix sort compare function in strategies

[stormslowly]

i am use mongoose 3.8.23 (a old version) the related mongodb native is "1.4.28".
we have 10host replset mongodb, whatever strategy we use the load is extremely un-balance.

after we dig in, we find the sort compare function return a boolean not a number so it will result in unsorted after call sort

e.g

b= [1,2,10,3,78,8,90,11,3,20,7,4,2]
b.sort(function(x,y){return x>y})
// b = [ 90,1,11,2,2,3,3,4,7,8,10,20,78]

after fix that the load is balanced to DBs.

But i can not found the 1.x.x related branch, and there is no strategies in 2.x.x
so i do PR in this branch.

if there are some proper place to do the PR just let me know.

[christkv]

you want to merge against the master branch.

[stormslowly]

@christkv Thanks you will do it on master branch