Java GC bugfix

This pull request adds two Buffer references to the
Request class. Whenever a request object is created
that has associated buffers, the buffers should be added
to these references so the java garbage collector does
not dispose of the buffers prematurely.

This is a more robust expansion on the idea first proposed by
@ggouaillardet, and further discussed in #369.

Fixes #369

Manual cherry-pick from d363b5d

Signed-off-by: Nathaniel Graham <[email protected]>

Author: Nathaniel Graham