CDAP-693 Adding the ability to run hive queries on streams.

This includes various changes to make this possible.

Streams are now created when an application is deployed
instead of lazily created when the first event is written to it.
If explore is enabled when a stream is created, an external
hive table is created using a custom HiveStorageHandler for
streams. This required some refactoring around the stream input
format, as our mapreduce code uses the newer mapreduce apis, but
the hive code uses the old mapred apis.
I don't think it'd be better, at least by returning 404 we give some information to the client  as to how his request was dealt with. Even if most clients will deal with 404 the same way they deal with a 200 here, we never know, there might be one client for which the info that the stream doesn't exist is useful.