net: Add ReadHalf::{peek,poll_peak}

[kleimkuhler]

Motivation

Closes #2136

Solution

The &mut self requirements for TcpStream methods ensure that there are at
most two tasks using the stream--one for reading and one for writing.

TcpStream::split allows two separate tasks to hold a reference to a single
TcpStream. TcpStream::{peek,poll_peek} only poll for read readiness, and
therefore are safe to use with a ReadHalf.

Instead of duplicating TcpStream::poll_peek, a private method is now used by
both poll_peek methods that uses the fact that only a &TcpStream is
required.

Signed-off-by: Kevin Leimkuhler kevin@kleimkuhler.com