feat(sync): headers commit threshold (#296)

* headers stream init

* fix tests

* return header if available regardless of control flow

* proper stream termination & docs

* upd headers stage to consume stream

* adjust response validation for stream

* use cursor.insert for headers

* wrap poll_next in a loop to bypass poking waker

* fix typo

* fix last td lookup

* Apply suggestions from code review

Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

* misc

* remove waker ref

* dedup response handling logic

* clippy

* add docs to poll

Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

crates/primitives/src/header.rs

@@ -266,6 +266,11 @@ impl SealedHeader {
     pub fn hash(&self) -> BlockHash {
         self.hash
     }
+
+    /// Return the number hash tuple.
+    pub fn num_hash(&self) -> (BlockNumber, BlockHash) {
+        (self.number, self.hash)
+    }
 }
 
 /// Represents the direction for a headers request depending on the `reverse` field of the request.