make pruned_bank channel unbonded.

In kin-sim, we found that bounded channel causes halt for account
background services. As the number of accounts grows, the time for
pruning and cleaning increases, which would leads to longer intervals
between the pruning of deaded bank slots. With 1.7B accounts, we will
exceed the 10K bounded channel threshold that causes halt of account
back ground services. Without pruning, the node will eventually run out
of memory.

core/src/validator.rs

@@ -1477,7 +1477,6 @@ fn load_blockstore(
     // is processing the dropped banks from the `pruned_banks_receiver` channel.
     let pruned_banks_receiver =
         AccountsBackgroundService::setup_bank_drop_callback(bank_forks.clone());
-
     {
         let hard_forks: Vec<_> = bank_forks
             .read()

runtime/src/accounts_background_service.rs

@@ -53,8 +53,6 @@ pub type DroppedSlotsReceiver = Receiver<(Slot, BankId)>;
 
 /// interval to report bank_drop queue events: 60s
 const BANK_DROP_SIGNAL_CHANNEL_REPORT_INTERVAL: u64 = 60_000;
-/// maximum drop bank signal queue length
-const MAX_DROP_BANK_SIGNAL_QUEUE_SIZE: usize = 10_000;
 
 /// Bank drop signal queue events
 #[allow(dead_code)]
@@ -609,8 +607,7 @@ impl AccountsBackgroundService {
     pub fn setup_bank_drop_callback(bank_forks: Arc<RwLock<BankForks>>) -> DroppedSlotsReceiver {
         assert_eq!(bank_forks.read().unwrap().banks().len(), 1);
 
-        let (pruned_banks_sender, pruned_banks_receiver) =
-            crossbeam_channel::bounded(MAX_DROP_BANK_SIGNAL_QUEUE_SIZE);
+        let (pruned_banks_sender, pruned_banks_receiver) = crossbeam_channel::unbounded();
         {
             let root_bank = bank_forks.read().unwrap().root_bank();
             root_bank.set_callback(Some(Box::new(