External spilling - generalize batching logic

The existing implementation consists of a hack for Kryo specifically and only works for LZF compression. Introducing an intermediate batch-level stream takes care of pre-fetching and other arbitrary behavior of higher level streams in a more general way.

Author: Andrew Or <andrewor14@gmail.com>

== Merge branch commits ==

commit 3ddeb7ef89a0af2b685fb5d071aa0f71c975cc82
Author: Andrew Or <andrewor14@gmail.com>
Date:   Wed Feb 5 12:09:32 2014 -0800

    Also privatize fields

commit 090544a87a0767effd0c835a53952f72fc8d24f0
Author: Andrew Or <andrewor14@gmail.com>
Date:   Wed Feb 5 10:58:23 2014 -0800

    Privatize methods

commit 13920c918efe22e66a1760b14beceb17a61fd8cc
Author: Andrew Or <andrewor14@gmail.com>
Date:   Tue Feb 4 16:34:15 2014 -0800

    Update docs

commit bd5a1d7350467ed3dc19c2de9b2c9f531f0e6aa3
Author: Andrew Or <andrewor14@gmail.com>
Date:   Tue Feb 4 13:44:24 2014 -0800

    Typo: phyiscal -> physical

commit 287ef44e593ad72f7434b759be3170d9ee2723d2
Author: Andrew Or <andrewor14@gmail.com>
Date:   Tue Feb 4 13:38:32 2014 -0800

    Avoid reading the entire batch into memory; also simplify streaming logic

    Additionally, address formatting comments.

commit 3df700509955f7074821e9aab1e74cb53c58b5a5
Merge: a531d2e 164489d
Author: Andrew Or <andrewor14@gmail.com>
Date:   Mon Feb 3 18:27:49 2014 -0800

    Merge branch 'master' of github.com:andrewor14/incubator-spark

commit a531d2e347acdcecf2d0ab72cd4f965ab5e145d8
Author: Andrew Or <andrewor14@gmail.com>
Date:   Mon Feb 3 18:18:04 2014 -0800

    Relax assumptions on compressors and serializers when batching

    This commit introduces an intermediate layer of an input stream on the batch level.
    This guards against interference from higher level streams (i.e. compression and
    deserialization streams), especially pre-fetching, without specifically targeting
    particular libraries (Kryo) and forcing shuffle spill compression to use LZF.

commit 164489d6f176bdecfa9dabec2dfce5504d1ee8af
Author: Andrew Or <andrewor14@gmail.com>
Date:   Mon Feb 3 18:18:04 2014 -0800

    Relax assumptions on compressors and serializers when batching

    This commit introduces an intermediate layer of an input stream on the batch level.
    This guards against interference from higher level streams (i.e. compression and
    deserialization streams), especially pre-fetching, without specifically targeting
    particular libraries (Kryo) and forcing shuffle spill compression to use LZF.