Ref count stream state and release when final (#73)

Previously, stream state was never released so that long-lived connections
leaked memory.

Now, stream states are reference-counted and freed from the stream slab
when complete.  Locally reset streams are retained so that received frames
may be ignored.

Cargo.toml

@@ -24,6 +24,7 @@ log = "0.3.8"
 fnv = "1.0.5"
 slab = "0.4.0"
 string = { git = "https://github.com/carllerche/string" }
+ordermap = "0.2"
 
 [dev-dependencies]