Simplify ReadWrite interface (#530)

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 They've been replaced by `NoopStream` for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.

examples/mbedtls/mbedtls_connector.rs

@@ -116,9 +116,6 @@ impl ReadWrite for MbedTlsStream {
     fn socket(&self) -> Option<&TcpStream> {
         None
     }
-    fn is_poolable(&self) -> bool {
-        true
-    }
 }
 
 impl io::Read for MbedTlsStream {