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/events/events_client.rs

@@ -58,10 +58,10 @@ where
         // return a Future that's Send + 'static, rather than tied to &self.
         let client = self.client.clone();
         async move {
-            // If a block ref isn't provided, we'll get the latest best block to use.
+            // If a block ref isn't provided, we'll get the latest finalized block to use.
             let block_ref = match block_ref {
                 Some(r) => r,
-                None => client.backend().latest_best_block_ref().await?,
+                None => client.backend().latest_finalized_block_ref().await?,
             };
 
             let event_bytes = get_event_bytes(client.backend(), block_ref.hash()).await?;