[SPARK-10810] [SPARK-10902] [SQL] Improve session management in SQL

This PR improve the sessions management by replacing the thread-local based to one SQLContext per session approach, introduce separated temporary tables and UDFs/UDAFs for each session.

A new session of SQLContext could be created by:

1) create an new SQLContext
2) call newSession() on existing SQLContext

For HiveContext, in order to reduce the cost for each session, the classloader and Hive client are shared across multiple sessions (created by newSession).

CacheManager is also shared by multiple sessions, so cache a table multiple times in different sessions will not cause multiple copies of in-memory cache.

Added jars are still shared by all the sessions, because SparkContext does not support sessions.

cc marmbrus yhuai rxin

Author: Davies Liu <davies@databricks.com>

Closes #8909 from davies/sessions.