Fix deadlock in SoftReferenceCache apply function.

When we find a stale SoftReference, release the read lock, then acquire
the write lock, double-check that the reference is still stale, and then
finally remove the entry from the cache.

core/util/src/main/scala/net/liftweb/util/SoftReferenceCache.scala

@@ -118,15 +118,28 @@ class SoftReferenceCache[K, V](cacheSize: Int) {
    * @param key
    * @return Box[V]
    */
-  def apply(key: K): Box[V] = lock(readLock) {
-    Box.!!(cache.get(key)) match {
-      case Full(value) =>
-        Box.!!(value.get) or {
-          remove(key);
-          Empty
+  def apply(key: K): Box[V] = {
+    val (doRemove:Boolean, retval:Box[V]) =
+      lock(readLock) {
+        Box.!!(cache.get(key)) match {
+          case Full(value) =>
+            Box.!!(value.get).map((false, _)) openOr {
+              (true, Empty)
+            }
+          case _ => (false, Empty)
         }
-      case _ => Empty
+      }
+
+    if (doRemove) {
+      lock(writeLock) {
+        val value = cache.get(key)
+
+        if (value != null && value.get == null)
+          remove(key)
+      }
     }
+
+    retval
   }
 
   /**