Take SparkConf in constructor of Serializer subclasses
Fix AbstractMethodError by inlining zip{Edge,Vertex}Partitions

The zip{Edge,Vertex}Partitions methods created doubly-nested closures
and passed them to zipPartitions. For some reason this caused an
AbstractMethodError when zipPartitions tried to invoke the closure. This
commit works around the problem by inlining these methods wherever they
are called, eliminating the doubly-nested closure.