…der` (#3006)

fix incomplete changes introduced from #2828

Previously, `to_bytes` would reserve extra space if after two chunks,
there was more remaining. It used to reserve however much space the peer
advertized.

This changes now only reserves up to ~16kb. This way, a slow message
with a big body doesn't reserve so much memory, until the data has
actually been received.

The existing warning to check for a length before calling the function
is still the best approach.

…3138)