Hash collision in mapreduce NIF

As per Aliaksey Artamonau comments on CBSE-2489

Potential lock-up

The core of the problem is here: http://src.couchbase.org/source/xref/4.1.0/couchdb/src/mapreduce/mapreduce.erl#53. This Ref value is then used in mapreduce_nif to index the contexts: http://src.couchbase.org/source/xref/4.1.0/couchdb/src/mapreduce/mapreduce_nif.cc#529. The problem is that phash2 is in no way collision-resistant. For example, here are two references that hash to the same value:

Hash: 107046579

Colliding refs: #Ref<0.0.0.23609> #Ref<0.0.0.19771>

What this means is that there's a chance that some context will get lost and will not abide by the javascript execution timeout.