Pre-allocate buffer for serialization

[abhinav]

It's easy to calculate the final size of the serialized object in advance. We should do that when serializing and allocate the write buffer with that size in advance so that we don't end up resizing it later.

The Protocol could expose a function that allows you to calculate the serialized size of an object, or if that specific protocol doesn't have guarantees around that, -1.

[abhinav]

CC @recht

Let me know if the serialized size thing becomes an issue and I'll prioritize this.