Improve cache

[cornelinux]

Implement last_bind and first_bind. This is to cope with the "re_binds" of an application like owncloud.

last_bind

Specify how long the last bind may be over. If last_bind is over, the cache will be emptied. This could be several minutes. If a new bind arrives, the old password may be used an last_bind would be reset.

first_bind

This specified how long the initial bind is allowed to be over.
If this time runs up, the cache will be emptied.

[fredreichbier]

As the rebind interval in owncloud is now configurable, I'm setting this to low priority.
However, this could still be interesting e.g. for odoo.

[cornelinux]

let's wait and see!