Don't use LRUCache as context cache to allow to clean it

References to the cursor cache are kept on BrowseRecord, we must be sure that
this cache will be cleaned.

review272001

trytond/backend/database.py

@@ -104,8 +104,7 @@ class CursorInterface(object):
     IN_MAX = 1000
 
     def __init__(self):
-        from trytond.cache import LRUDict
-        self.cache = LRUDict(CONTEXT_CACHE_SIZE)
+        self.cache = {}
 
     def get_cache(self, context=None):
         '''