Introduce KeyedLocker, a nice concurrency abstraction for managing lots of mutexes, and RefCountedMultisetKeyedLocker, an efficient implementation of this abstraction.

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diff --git a/src/main/java/com/google/devtools/build/lib/concurrent/KeyedLocker.java b/src/main/java/com/google/devtools/build/lib/concurrent/KeyedLocker.java
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+// Copyright 2015 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.concurrent;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/** A keyed store of locks. */
+@ThreadSafe
+public interface KeyedLocker<K> {
+  /** Used to yield access to the implicit lock granted by {@link #lock}. */
+  @ThreadSafe
+  interface AutoUnlocker extends AutoCloseable {
+    /**
+     * If this was returned by {@code lock(k)}, yields exclusive access to {@code k}.
+     *
+     * <p>This method should be called at most once, and may only be called by the same thread that
+     * acquired the {@link AutoUnlocker} via {@link #lock}. Implementations are free to do anything
+     * if this is violated.
+     */
+    @Override
+    void close();
+  }
+
+  /**
+   * Blocks the current thread until it has exclusive access to do things with {@code k} and
+   * returns a {@link AutoUnlocker} instance for yielding the implicit lock. The intended usage
+   * is:
+   *
+   * <pre>
+   * {@code
+   * try (AutoUnlocker unlocker = locker.lock(k)) {
+   *   // Your code here.
+   * }
+   * }
+   * </pre>
+   */
+  AutoUnlocker lock(K key);
+}
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