Metadata: resolve raw generic class compiler warning.

.../driver-core/src/main/java/com/datastax/driver/core/Metadata.java:[39,40] [rawtypes] found raw type: Token
[WARNING]   missing type arguments for generic class Token<T>
  where T is a type-variable:
    T extends Token<T> declared in class Token
.../driver-core/src/main/java/com/datastax/driver/core/Metadata.java:[181,8] [rawtypes] found raw type: TokenMap