Previously, ReadWrite had methods `is_poolable` and `written_bytes`, which
were solely for the use of unittests.
This replaces `written_bytes` and `TestStream` with a `struct Recorder`
that implements `ReadWrite` and allows unittests to access its recorded
bytes via an `Arc<Mutex<Vec<u8>>>`. It eliminates `is_poolable`; it's fine
to pool a Stream of any kind.
The new `Recorder` also has some convenience methods that abstract away
boilerplate code from many of our unittests.
I got rid of `Stream::from_vec` and `Stream::from_vec_poolable` because
they depended on `TestStream`. They've been replaced by `NoopStream` for
the pool.rs tests, and `ReadOnlyStream` for constructing `Response`s from
`&str` and some test cases.
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