Fixed several bugs exposed by setting the cache size to an

unreasonably small value.  Had to make a compromise to avoid a deadlock
though.  read/write/append/truncate will now be able to exceed the cache size
limit temporarily.  The cache will be reconciled at the end of the respective
operation.

Ex, given a cache of 100MB, and a read() of 500MB, all 500MB
of data being read will stay in the cache until it is read, then
400MB of it will be evicted.  Same on the write side.

src/unit_tests.cpp

@@ -15,6 +15,7 @@
 #include "Replicator.h"
 #include "S3Storage.h"
 #include "Utilities.h"
+#include "Synchronizer.h"
 
 #include <iostream>
 #include <stdlib.h>