chainHead based backend implementation (#1161)

* add follow_stream impl

* follow_stream_unpin first draft

* add tests for follow_stream_unpin

* more tests and fixes for follow_stream_unpin

* first pass follow_stream_driver

* follow_stream_driver: add tests, fix things, buffer events from last finalized

* First pass finishing Backend impl

* Fix test compile issues

* clippy fixes

* clippy fix and consistify light_client

* revert lightclient tweak

* revert other lightclient thing

* cargo fmt

* start testing unstable backend behind feature flag

* more test fixes and move test-runtime metadata path just incase

* fix compile error

* ensure transaction progress stream actually used and fix another test

* cargo fmt

* CI tweak

* improve some comments and address some feedback bits

* update CI to use our own nightly binary

* wait for finalized block perhaps

subxt/src/client/online_client.rs

@@ -106,7 +106,7 @@ impl<T: Config> OnlineClient<T> {
     /// Construct a new [`OnlineClient`] by providing an underlying [`Backend`]
     /// implementation to power it. Other details will be obtained from the chain.
     pub async fn from_backend<B: Backend<T>>(backend: Arc<B>) -> Result<OnlineClient<T>, Error> {
-        let latest_block = backend.latest_best_block_ref().await?;
+        let latest_block = backend.latest_finalized_block_ref().await?;
 
         let (genesis_hash, runtime_version, metadata) = future::join3(
             backend.genesis_hash(),
@@ -437,7 +437,7 @@ impl<T: Config> RuntimeUpdaterStream<T> {
             Err(err) => return Some(Err(err)),
         };
 
-        let latest_block_ref = match self.client.backend().latest_best_block_ref().await {
+        let latest_block_ref = match self.client.backend().latest_finalized_block_ref().await {
             Ok(block_ref) => block_ref,
             Err(e) => return Some(Err(e)),
         };