Redesign pooling mechanic (#565)

Introduce PoolReturner, a handle on an agent and a PoolKey that is
capable of returning a Stream to a Pool. Make Streams keep track of
their own PoolReturner, instead of having PoolReturnRead keep track of
that information.

For the LimitedRead code path, get rid of PoolReturnRead. Instead,
LimitedRead is responsible for returning its Stream to the Pool after
its second-to-last read. In other words, LimitedRead will return the
stream if the next read is guaranteed to return Ok(0).

Constructing a LimitedRead of size 0 is always wrong, because we could
always just return the stream immediately. Change the size argument to
NonZeroUsize to enforce that.

Remove the Done trait, which was only used for LimitedRead. It was used
to try and make sure we returned the stream to the pool on exact reads,
but was not reliable.

This does not yet move the ChunkDecoder code path away from
PoolReturnRead. That requires a little more work.

Part 1 of #559.  Fixes #555.

src/test/mod.rs

@@ -1,4 +1,5 @@
 use crate::error::Error;
+use crate::pool::PoolReturner;
 use crate::stream::{remote_addr_for_test, ReadOnlyStream, Stream};
 use crate::unit::Unit;
 use crate::ReadWrite;
@@ -55,6 +56,7 @@ pub(crate) fn make_response(
     Ok(Stream::new(
         ReadOnlyStream::new(buf),
         remote_addr_for_test(),
+        PoolReturner::none(),
     ))
 }
 
@@ -103,6 +105,7 @@ impl Recorder {
         Stream::new(
             TestStream::new(cursor, self.clone()),
             remote_addr_for_test(),
+            PoolReturner::none(),
         )
     }
 }