Initial code to test connections before handing them out

The goal is to fix issues where we have dead connections in the pool
which are being handed out. In particular, when all the connections in
the pool are dead it can easily exhaust a client's retry budget.x

modules/core/shared/src/main/scala/util/Pool.scala

@@ -95,12 +95,19 @@ object Pool {
               case _ => ref.update { case (os, ds) => (os :+ None, ds) }
             }
 
+            def createAlloc: F[(A, F[Unit])] = Concurrent[F].onError(rsrc(Tracer[F]).allocated)(restore)
+
             // Here we go. The cases are a full slot (done), an empty slot (alloc), and no slots at
             // all (defer and wait).
             ref.modify {
-              case (Some(a) :: os, ds) => ((os, ds), a.pure[F])
-              case (None    :: os, ds) => ((os, ds), Concurrent[F].onError(rsrc(Tracer[F]).allocated)(restore))
-              case (Nil,           ds) =>
+              case (Some(a) :: os, ds) =>
+                ((os, ds), recycler(a._1).attempt.flatMap {
+                  case Right(true) => a.pure[F]
+                  case Left(_) | Right(false) => dispose(a) *> createAlloc
+                })
+              case (None :: os, ds) =>
+                ((os, ds), createAlloc)
+              case (Nil, ds) =>
                 val cancel = ref.flatModify { // try to remove our deferred
                   case (os, ds) =>
                     val canRemove = ds.contains(d)