MAHOUT-1578 Optimizations in matrix serialization

MAHOUT-1578: Optimizations in matrix serialization
is that where the major problem was? is that because assignment of sequential vector to sequential vector is that slow? or this is an assignment of random vector to a sequential vector? (sequential to sequential actually should be ok methinks). 

Anyway I don't see any immediate problems, and the quality of your work usually doesn't require any deep scrutiny, so i'd say ship it. Actually the sooner the better, because i am very close to actually give it all a good spanking