LinkageError in IdentityStore.validate()

The use of `MethodHandles` in `IdentityStore.validate()` may lead to linkage errors.

This issue may occur in

```java
MethodHandles.lookup()
.bind(this, "validate", methodType(CredentialValidationResult.class, credential.getClass()))
```

when redeploying a web application.

In this case, `methodType()` may return a stale reference caused by the internal use of weak references which may still hang around if the classloader of the original deployment has not yet been garbage collected.

I ran into this problem when executing an Arquillian test suite on a remote server, where every test class gives rise to a new deployment. Observed on Java 8 as well as Java 11.

See https://github.com/hwellmann/method-type-leak for a self-contained test case.

Possible workaround:

Override `IdentityStore.validate(Credential)` in your custom class and downcast the `Credential`argument to the concrete type.