Fix FutureAsyncCache

[kitlangton]

The ZIO Async Cache never actually cached anything. So we removed it for now, potentially eventually replacing it with zio-cache. For now we simply replaced it with the AsyncFutureCache, fixing some correctness issues there as well. Previously, it was unsafely casting the ExecutionContext to an Executor, which happened to work with what was being passed in before, but broke when the ExecutionContext was ZIO's internal Executor.

[deusaquilus]

Note that the andThen part in this prepareAsync (after the asScala) was always wrong:

trait AsyncFutureCache { this: CassandraSession =>
  lazy val asyncCache = new PrepareStatementCache[Future[PreparedStatement]](preparedStatementCacheSize)
  def prepareAsync(cql: String)(implicit executionContext: ExecutionContext): Future[BoundStatement] =
    asyncCache(cql)(stmt => session.prepareAsync(stmt).asScala andThen {
      case Failure(_) => asyncCache.invalidate(stmt)
    }).map(_.bind())
}

The problem here is that the result of session.prepareAsync(stmt).asScala is not a promise, it's a future so the partial function it's being fed into here always fails (i.e. since Future[PreparedStatement] is never a scala.util.Failure.
That means that this cache has not been working in the first place.