Change Blocking to use Sync#interruptible, not blocking (#1126)

This is to avoid being unable to cancel code that is inside blocking
section. At the moment it is possible for user-code to not cancel
immediately, in which case it will block until an internal Kafka
max-block period is reached (by default 1 minute).

Changing to use `interruptible` means the user-code returns immediately
on cancellation, and the underlying operation in Kafka should be
cancelled.

modules/core/src/main/scala/fs2/kafka/internal/Blocking.scala

@@ -20,7 +20,7 @@ private[kafka] trait Blocking[F[_]] {
 private[kafka] object Blocking {
 
   def fromSync[F[_]: Sync]: Blocking[F] = new Blocking[F] {
-    override def apply[A](a: => A): F[A] = Sync[F].blocking(a)
+    override def apply[A](a: => A): F[A] = Sync[F].interruptible(a)
   }
 
   def fromExecutionContext[F[_]](ec: ExecutionContext)(implicit F: Async[F]): Blocking[F] =