Parameterize the buffer for marshal/unmarshal.

We are not reusing buffers yet.  This could introduce problems,
so the behavior is disabled for now.

Cursory benchmark data:
- Marshal for 10,000 samples: -30% overhead.
- Unmarshal for 10,000 samples: -15% overhead.

Change-Id: Ib006bdc656af45dca2b92de08a8f905d8d728cac