Zero-copy networking

Linux has somewhat recently introduced zero-copy networking primitives that we might be able to take advantage of.

Sending: https://lwn.net/Articles/726917/
Receiving: https://lwn.net/Articles/752188/

Commentary suggests that it's only helpful for some workloads dealing with large amounts of data, and gives feedback as to when its not being helpful or results in a copy anyway. But it should be easy for us to implement as it should only change arena lifetime, I think.

No source files, tests, or entry points are named. Start by reading the linked LWN articles, then locate FoundationDB's Linux networking send and receive paths and determine how arena lifetime would need to change. Done should include zero-copy behavior where beneficial and evidence for workloads where it is not.

Written by the indexing model from the issue text.