The previous mechanism could take over a minute with a big cache.
It did two checks:
    1. (expensive check) Compare pair-wise all keys loaded, and find those
       that are equal even though they have a different hash.
    2. (cheap one) Ensure that unpickling a pickled key yields a key that is
       equal to the original one.
Those checks are now replaced by a new check that is performed each time a new
key is saved in the cache. We reload the pickled KeyData object and ensure
that it contains exactly one unpickled key equal to the key originally saved.
It is obvious it will catch errors that would have been caught by check #2.
It will also catch errors that would have been caught by check #1 because if
two keys are equal, they must yield a similar module hash, and thus they will
be stored in the same KeyData object: if they have a different key hash, they
will both appear in the 'keys' set of the KeyData object, and thus the new
check will complain there is more than one key found to be equal to the one we
just saved.

NB: Also removed a couple TODOs now that I understand better why some code was
written that way.

The old behaviour used to be that the id is added only if the name of the
apply node was too long and not when the name was short.

I have changed the gradients trough a cast to be defined if and only if both
the input and output are of continuous types.

Duplicated modules are now deleted with 'theano-cache clear' as well as automatically once they are old enough

The 'filter' method of a ConfigParam object is not called anymore on the
default value when the ConfigParam is created.

Added a test case to test this new behavior.

Avoid crash in clear_old when the path to the compiledir is not exactly the same as when the module was created

- 'theano-cache clear' will now delete cache entries containing pickled files
that cannot be unpickled.
- Replaced some manual calls to shutil.rmtree by calls to _rmtree to
standardize cache directory deletion.
- Cache entries associated to KeyData objects whose keys are broken should
now be cleared if the module file is gone.
- More comprehensive cleanup of the entry_from_key, module_hash_to_key_data
and loaded_key_pkl dictionaries when deleting a cache entry.