avoid `Taskpool.spawn` within `{.async.}` for Nim 2.0 (#5393)

In Nim 2.0, attempting to use `Taskpool.spawn` inside `{.async.}` `proc`
leads to `Error: cannot generate destructor for generic type: Isolated`.

Add an intermediate wrapper `proc` that performs the `spawn` operation
to workaround the problem.

beacon_chain/gossip_processing/batch_validation.nim

@@ -228,6 +228,13 @@ proc batchVerifyTask(task: ptr BatchTask) {.nimcall.} =
 
   discard task[].signal.fireSync()
 
+proc spawnBatchVerifyTask(tp: Taskpool, task: ptr BatchTask) =
+  # Inlining this `proc` leads to compilation problems on Nim 2.0
+  # - Error: cannot generate destructor for generic type: Isolated
+  # Workaround: Ensure that `tp.spawn` is not used within an `{.async.}` proc
+  # Possibly related to: https://github.com/nim-lang/Nim/issues/22305
+  tp.spawn batchVerifyTask(task)
+
 proc batchVerifyAsync*(
     verifier: ref BatchVerifier, signal: ThreadSignalPtr,
     batch: ref Batch): Future[bool] {.async.} =
@@ -245,7 +252,7 @@ proc batchVerifyAsync*(
   let taskPtr = addr task
   doAssert verifier[].taskpool.numThreads > 1,
     "Must have at least one separate thread or signal will never be fired"
-  verifier[].taskpool.spawn batchVerifyTask(taskPtr)
+  verifier[].taskpool.spawnBatchVerifyTask(taskPtr)
   await signal.wait()
   task.ok.load()