Add model for new Last Modified system.

This will replace the current stuff in thing_utils.py. The idea is that
by storing the various last modified timestamps for a thing in a single
row together, we can save Cassandra round trips and seeks per request.

r2/r2/models/last_modified.py

@@ -0,0 +1,37 @@
+import datetime
+
+from pylons import g
+from pycassa.system_manager import ASCII_TYPE, DATE_TYPE
+
+from r2.lib.db import tdb_cassandra
+from r2.lib.utils import tup
+
+
+class LastModified(tdb_cassandra.View):
+    _use_db = True
+    _value_type = "date"
+    _connection_pool = "main"
+    _read_consistency_level = tdb_cassandra.CL.ONE
+    _extra_schema_creation_args = dict(key_validation_class=ASCII_TYPE,
+                                       default_validation_class=DATE_TYPE)
+
+    @classmethod
+    def touch(cls, fullname, names):
+        names = tup(names)
+        now = datetime.datetime.now(g.tz)
+        values = dict.fromkeys(names, now)
+        cls._set_values(fullname, values)
+        return now
+
+    @classmethod
+    def get(cls, fullname, name, touch_if_not_set=False):
+        try:
+            obj = cls._byID(fullname)
+        except tdb_cassandra.NotFound:
+            if touch_if_not_set:
+                time = cls.touch(fullname, name)
+                return time
+            else:
+                return None
+
+        return getattr(obj, name, None)