Pig should clone JobConf while creating JobContextImpl and TaskAttemptContextImpl in Hadoop23

There is change in the semantics of
JobContext::JobContext(Configuration, JobID). While in .20, the Config was
cloned, in .23 the Config is adopted (if it's a JobConf). This causes the same
Configuration instance to be written-to for different tables in the same job.

It would affect multi store commands in pig on Hadoop 23/2.0. The
cloning in HadoopShims was part of PIG-2578 but was reverted to other issues.