Do not clear cache in __del__

Prior to this change, running
`py.test --assert=rewrite test/test_integration.py` fails with errors
due to:

```py
cls = <class 'jedi.recursion.ExecutionRecursionDecorator'>

    @classmethod
    def cleanup(cls):
>       cls.parent_execution_funcs.pop()
E       IndexError: pop from empty list

recursion.py:127: IndexError
```

Similar errors occurred in travis occasionally:
- https://travis-ci.org/davidhalter/jedi/jobs/5449831
- https://travis-ci.org/davidhalter/jedi/jobs/5512538

I think this is because GC calls __del__ at random point during
actual execution of ExecutionRecursionDecorator.__call__.
As --assert=rewrite works by AST dynamically, I guess that it
is harder for Python's GC to clean up code and therefore GC
happens sometime later.  Although this is just a random guess,
as `tox -- --assert=rewrite` works with this patch now, I think
this is a good change.

jedi/api_classes.py

@@ -7,6 +7,7 @@ interesting information about completion and goto operations.
 import re
 import os
 import warnings
+import functools
 
 from _compatibility import unicode, next
 import cache
@@ -34,6 +35,18 @@ def _clear_caches():
     imports.imports_processed = 0
 
 
+def _clear_caches_after_call(func):
+    """
+    Clear caches just before returning a value.
+    """
+    @functools.wraps(func)
+    def wrapper(*args, **kwds):
+        result = func(*args, **kwds)
+        _clear_caches()
+        return result
+    return wrapper
+
+
 class BaseDefinition(object):
     _mapping = {'posixpath': 'os.path',
                'riscospath': 'os.path',