title: Limit subscription concurrency in Reactor
body: <p>I have an utility class centralizing Reactor's <code>Publisher</code>s to deal with Cassandra queries:</p>
  
  <pre><code>public Mono&lt;ResultSet&gt; execute(Statement statement) {
      return Mono.defer(() -&gt; Mono.fromFuture(FutureConverter
              .toCompletableFuture(session.executeAsync(statement)))
              .publishOn(Schedulers.elastic()));
  }
  </code></pre>
  
  <p>It works well expect it does not deal with back-pressure, which ends up with Cassandra pool exhaustion, consequently an error.
  Is there a way to to limit the concurrency of the subscriptions (putting them in a FIFO queue) to a given number (which will be match to the pool's size)?</p>
  
  <p>Thanks in advance.</p>