Simplify the chain_head.rs module (#409)

* Move the chainhead_follow task to another function

* Merge FollowSubscription into ChainHeadFollowTask

* Extract processing foreground message to separate function

* Move the subscription start functions as methods of ChainHeadFollowTask

* Fix generating a `chainHead_unstable_followEvent` event on unfollow

* Fix cyclic references in the chainHead sub-tasks

* Finish fixing cyclic references issues

* Move some fields to the struct

* Move subscription as a struct field

* PR link

* CHANGELOG entry for cyclic reference

* Remove CHANGELOG entry as still not fixed

* Properly fix cyclic reference

* Add back CHANGELOG entry

* Oops, fix bug

light-base/src/json_rpc_service/background.rs

@@ -36,9 +36,7 @@ use core::{
     time::Duration,
 };
 use futures::{lock::Mutex, prelude::*};
-use hashbrown::HashMap;
 use smoldot::{
-    chain::fork_tree,
     executor::{host, runtime_host},
     header,
     json_rpc::{self, methods, requests_subscriptions},
@@ -101,16 +99,6 @@ struct Background<TPlat: Platform> {
     printed_legacy_json_rpc_warning: atomic::AtomicBool,
 }
 
-struct FollowSubscription {
-    /// Tree of hashes of all the current non-finalized blocks. This includes unpinned blocks.
-    non_finalized_blocks: fork_tree::ForkTree<[u8; 32]>,
-
-    /// For each pinned block hash, the SCALE-encoded header of the block.
-    pinned_blocks_headers: HashMap<[u8; 32], Vec<u8>, fnv::FnvBuildHasher>,
-
-    runtime_subscribe_all: Option<runtime_service::SubscriptionId>,
-}
-
 pub(super) enum SubscriptionMessage {
     StopIfAllHeads {
         stop_request_id: (String, requests_subscriptions::RequestId),