Untangled cache flushing behavior s.t. it should no longer deadlock.

Now, something that grabs the sync or cache locks should never block
while holding the lock.  Need to review the code to make sure of that.

Also made the recursive mutexes non-recursive again.

src/Synchronizer.h

@@ -56,7 +56,7 @@ class Synchronizer : public boost::noncopyable
             int waiters;
             bool finished;
             boost::condition condvar;
-            void wait(boost::recursive_mutex *);
+            void wait(boost::mutex *);
             void notify();
         };
         
@@ -94,7 +94,7 @@ class Synchronizer : public boost::noncopyable
         
         boost::filesystem::path cachePath;
         boost::filesystem::path journalPath;
-        boost::recursive_mutex mutex;
+        boost::mutex mutex;
 };
 
 }