Provide more options for the AsyncExecutor

Allow the user to fine-tune the underlying thread pool of the AsyncExecutor

slick/src/main/scala/slick/util/AsyncExecutor.scala

@@ -3,6 +3,7 @@ package slick.util
 import java.io.Closeable
 import java.util.concurrent.atomic.AtomicInteger
 import java.util.concurrent._
+import scala.concurrent.duration._
 import scala.concurrent.{Promise, Future, ExecutionContext}
 import scala.util.control.NonFatal
 
@@ -23,7 +24,19 @@ object AsyncExecutor extends Logging {
     * @param name A prefix to use for the names of the created threads.
     * @param numThreads The number of threads in the pool.
     * @param queueSize The size of the job queue, 0 for direct hand-off or -1 for unlimited size. */
-  def apply(name: String, numThreads: Int, queueSize: Int): AsyncExecutor = {
+  def apply(name: String, numThreads: Int, queueSize: Int): AsyncExecutor = apply(name, numThreads, numThreads, queueSize)
+
+  /** Create an [[AsyncExecutor]] with a thread pool suitable for blocking
+    * I/O. New threads are created as daemon threads.
+    *
+    * @param name A prefix to use for the names of the created threads.
+    * @param minThreads The number of core threads in the pool.
+    * @param maxThreads The maximum number of threads in the pool.
+    * @param queueSize The size of the job queue, 0 for direct hand-off or -1 for unlimited size.
+    * @param keepAliveTime when the number of threads is greater than
+    *        the core, this is the maximum time that excess idle threads
+    *        will wait for new tasks before terminating.*/
+  def apply(name: String, minThreads: Int, maxThreads: Int, queueSize: Int, keepAliveTime: Duration = 1.minute): AsyncExecutor = {
     new AsyncExecutor {
       // Before init: 0, during init: 1, after init: 2, during/after shutdown: 3
       private[this] val state = new AtomicInteger(0)
@@ -45,7 +58,7 @@ object AsyncExecutor extends Logging {
             }
         }
         val tf = new DaemonThreadFactory(name + "-")
-        executor = new ThreadPoolExecutor(numThreads, numThreads, 1, TimeUnit.MINUTES, queue, tf)
+        executor = new ThreadPoolExecutor(minThreads, maxThreads, keepAliveTime.toMillis, TimeUnit.MILLISECONDS, queue, tf)
         if(!state.compareAndSet(1, 2)) {
           executor.shutdownNow()
           throw new IllegalStateException("Cannot initialize ExecutionContext; AsyncExecutor shut down during initialization")