mem: Create a separate class for the cache write buffer

This patch breaks out the cache write buffer into a separate class,
without affecting any stats. The goal of the patch is to avoid
encumbering the much-simpler write queue with the complex MSHR
handling. In a follow on patch this simplification allows us to
implement write combining.

The WriteQueue gets its own class, but shares a common ancestor, the
generic Queue, with the MSHRQueue.

src/mem/cache/SConscript

@@ -37,6 +37,8 @@ Source('cache.cc')
 Source('blk.cc')
 Source('mshr.cc')
 Source('mshr_queue.cc')
+Source('write_queue.cc')
+Source('write_queue_entry.cc')
 
 DebugFlag('Cache')
 DebugFlag('CachePort')