[IMP] api: avoid record.id indirections

The cache is quite time critical, as it is accessed millions times when reading
multiple fields on hundred of thousands of records.

Avoiding indirections and `if` statement when possible speeds up the cache
access time.

odoo/api.py

@@ -964,13 +964,13 @@ class Cache(object):
     def get(self, record, field):
         """ Return the value of ``field`` for ``record``. """
         key = record.env.cache_key(field)
-        value = self._data[key][field][record.id]
+        value = self._data[key][field][record._ids[0]]
         return value.get() if isinstance(value, SpecialValue) else value
 
     def set(self, record, field, value):
         """ Set the value of ``field`` for ``record``. """
         key = record.env.cache_key(field)
-        self._data[key][field][record.id] = value
+        self._data[key][field][record._ids[0]] = value
 
     def remove(self, record, field):
         """ Remove the value of ``field`` for ``record``. """