avoid some futures and copies in processing pipeline (#2157)

`addLastNoWait` does the same thing with less task scheduling and
copying

beacon_chain/eth2_processor.nim

@@ -79,13 +79,11 @@ proc updateHead*(self: var Eth2Processor, wallSlot: Slot) =
   # justified and finalized
   let
     oldFinalized = self.chainDag.finalizedHead.blck
-    oldHead = self.chainDag.head
 
   self.chainDag.updateHead(newHead, self.quarantine)
 
   # Cleanup the fork choice v2 if we have a finalized head
   if oldFinalized != self.chainDag.finalizedHead.blck:
-
     self.attestationPool[].prune()
 
 proc dumpBlock[T](
@@ -318,14 +316,19 @@ proc attestationValidator*(
   beacon_attestation_delay.observe(delay.toFloatSeconds())
 
   while self.attestationsQueue.full():
-    let dropped = self.attestationsQueue.popFirst()
-    doAssert dropped.finished, "popFirst sanity"
+    try:
       notice "Queue full, dropping attestation",
-      dropped = shortLog(dropped.read().v)
+        dropped = shortLog(self.attestationsQueue[0].v)
+      discard self.attestationsQueue.popFirstNoWait()
+    except CatchableError as exc:
+      raiseAssert "If queue is full, we have at least one item! " & exc.msg
 
   trace "Attestation validated"
-  traceAsyncErrors self.attestationsQueue.addLast(
+  try:
+    self.attestationsQueue.addLastNoWait(
       AttestationEntry(v: attestation, attesting_indices: v.get()))
+  except CatchableError as exc:
+    raiseAssert "We just checked that queue is not full! " & exc.msg
 
   ValidationResult.Accept
 
@@ -361,15 +364,20 @@ proc aggregateValidator*(
   beacon_aggregate_delay.observe(delay.toFloatSeconds())
 
   while self.aggregatesQueue.full():
-    let dropped = self.aggregatesQueue.popFirst()
-    doAssert dropped.finished, "popFirst sanity"
+    try:
       notice "Queue full, dropping aggregate",
-      dropped = shortLog(dropped.read().v)
+        dropped = shortLog(self.aggregatesQueue[0].v)
+      discard self.aggregatesQueue.popFirstNoWait()
+    except CatchableError as exc:
+      raiseAssert "We just checked that queue is not full! " & exc.msg
 
   trace "Aggregate validated"
-  traceAsyncErrors self.aggregatesQueue.addLast(AggregateEntry(
+  try:
+    self.aggregatesQueue.addLastNoWait(AggregateEntry(
       v: signedAggregateAndProof.message.aggregate,
       attesting_indices: v.get()))
+  except CatchableError as exc:
+    raiseAssert "We just checked that queue is not full! " & exc.msg
 
   ValidationResult.Accept