Experimental support for mutable RDDs in delta updates

This breaks correctness in general, but is OK on our benchmarks.

I benchmarked it for connected components on uk-union, and it provides a 57% speedup. I ran 10 trials of 20 iterations each on 16 m2.4xlarge machines, coalescing the input to 64 partitions. Original runtime was 870 +/- 31 s, and with this PR it was reduced to 369 +/- 6 s.