tonic-build: option to emit Arc<Self> as server receiver (#1352)

This implements an option to emit `Arc<Self>` instead of `&self` in
server traits. This enables implementor to reference `Self` data for
indefinite duration, which may be useful for streaming requests.

Fixes: #1351

tests/use_arc_self/build.rs

@@ -0,0 +1,6 @@
+fn main() {
+    tonic_build::configure()
+        .use_arc_self(true)
+        .compile(&["proto/test.proto"], &["proto"])
+        .unwrap();
+}