Do not reset cache on commit and clear

When a transaction is committed indeed a new transaction is started. So we do
not need to use a transaction cache just because a commit was sent because the
started_at is reset to a higher time.
This same logic applies to a rollback.

issue10528
review365521003

trytond/cache.py

@@ -250,7 +250,7 @@ class MemoryCache(BaseCache):
     @classmethod
     def commit(cls, transaction):
         table = Table(cls._table)
-        reset = cls._reset.setdefault(transaction, set())
+        reset = cls._reset.pop(transaction, None)
         if not reset:
             return
         database = transaction.database
@@ -303,10 +303,7 @@ class MemoryCache(BaseCache):
 
     @classmethod
     def rollback(cls, transaction):
-        try:
-            cls._reset[transaction].clear()
-        except KeyError:
-            pass
+        cls._reset.pop(transaction, None)
 
     @classmethod
     def drop(cls, dbname):