Change ReserveCapacity to expanded FlowControl type (#423)

- Adds `FlowControl::available_capacity` method.
- Adds `FlowControl::used_capacity` method.

src/server.rs

@@ -118,7 +118,7 @@
 use crate::codec::{Codec, RecvError, UserError};
 use crate::frame::{self, Pseudo, PushPromiseHeaderError, Reason, Settings, StreamId};
 use crate::proto::{self, Config, Prioritized};
-use crate::{PingPong, RecvStream, ReleaseCapacity, SendStream};
+use crate::{FlowControl, PingPong, RecvStream, SendStream};
 
 use bytes::{Buf, Bytes, IntoBuf};
 use http::{HeaderMap, Request, Response};
@@ -408,7 +408,7 @@ where
         if let Some(inner) = self.connection.next_incoming() {
             log::trace!("received incoming");
             let (head, _) = inner.take_request().into_parts();
-            let body = RecvStream::new(ReleaseCapacity::new(inner.clone_to_opaque()));
+            let body = RecvStream::new(FlowControl::new(inner.clone_to_opaque()));
 
             let request = Request::from_parts(head, body);
             let respond = SendResponse { inner };
@@ -427,14 +427,14 @@ where
     ///
     /// If `size` is less than the current value, nothing will happen
     /// immediately. However, as window capacity is released by
-    /// [`ReleaseCapacity`] instances, no `WINDOW_UPDATE` frames will be sent
+    /// [`FlowControl`] instances, no `WINDOW_UPDATE` frames will be sent
     /// out until the number of "in flight" bytes drops below `size`.
     ///
     /// The default value is 65,535.
     ///
-    /// See [`ReleaseCapacity`] documentation for more details.
+    /// See [`FlowControl`] documentation for more details.
     ///
-    /// [`ReleaseCapacity`]: ../struct.ReleaseCapacity.html
+    /// [`FlowControl`]: ../struct.FlowControl.html
     /// [library level]: ../index.html#flow-control
     pub fn set_target_window_size(&mut self, size: u32) {
         assert!(size <= proto::MAX_WINDOW_SIZE);
@@ -591,11 +591,11 @@ impl Builder {
     /// flow control for received data.
     ///
     /// The initial window of a stream is used as part of flow control. For more
-    /// details, see [`ReleaseCapacity`].
+    /// details, see [`FlowControl`].
     ///
     /// The default value is 65,535.
     ///
-    /// [`ReleaseCapacity`]: ../struct.ReleaseCapacity.html
+    /// [`FlowControl`]: ../struct.FlowControl.html
     ///
     /// # Examples
     ///
@@ -625,11 +625,11 @@ impl Builder {
     /// for received data.
     ///
     /// The initial window of a connection is used as part of flow control. For more details,
-    /// see [`ReleaseCapacity`].
+    /// see [`FlowControl`].
     ///
     /// The default value is 65,535.
     ///
-    /// [`ReleaseCapacity`]: ../struct.ReleaseCapacity.html
+    /// [`FlowControl`]: ../struct.FlowControl.html
     ///
     /// # Examples
     ///