[SPARK-6405] Limiting the maximum Kryo buffer size to be 2GB.

Kryo buffers are backed by byte arrays, but primitive arrays can only be
up to 2GB in size. It is misleading to allow users to set buffers past
this size.

Author: mcheah <mcheah@palantir.com>

Closes #5218 from mccheah/feature/limit-kryo-buffer and squashes the following commits:

1d6d1be [mcheah] Fixing numeric typo
e2e30ce [mcheah] Removing explicit int and double type to match style
09fd80b [mcheah] Should be >= not >. Slightly more consistent error message.
60634f9 [mcheah] [SPARK-6405] Limiting the maximum Kryo buffer size to be 2GB.