Enforce monotonic stream IDs for push promises (#275)

Previously, monotonic stream IDs (spec 5.1.1) for push promises were not
enforced. This was due to push promises going through an entirely
separate code path than normally initiated streams.

This patch unifies the code path for initializing streams via both
HEADERS and PUSH_PROMISE. This is done by first calling `recv.open` in
both cases.

Closes #272

src/proto/streams/mod.rs

@@ -10,6 +10,7 @@ mod stream;
 mod streams;
 
 pub(crate) use self::prioritize::Prioritized;
+pub(crate) use self::recv::Open;
 pub(crate) use self::store::Key;
 pub(crate) use self::streams::{StreamRef, OpaqueStreamRef, Streams};