[SPARK-19084][SQL] Ensure context class loader is set when initializing Hive.

A change in Hive 2.2 (most probably HIVE-13149) causes this code path to fail,
since the call to "state.getConf.setClassLoader" does not actually change the
context's class loader. Spark doesn't yet officially support Hive 2.2, but some
distribution-specific metastore client libraries may have that change (as certain
versions of CDH already do), and this also makes it easier to support 2.2 when it
comes out.

Tested with existing unit tests; we've also used this patch extensively with Hive
metastore client jars containing the offending patch.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #17154 from vanzin/SPARK-19804.