Add close() to URLOperations

[mih]

Right now all handler implementations only support "one-off" operations, i.e. each call performs everything from scratch. This is simple, but also costly in the case of handlers that have significant setup costs. One example is SSH which requires getting a shell per operation.

It would make sense to "cache" setup stages to allow for efficient implementations. This also means that we need to support a cleanup handler, such as close() to let implementations free up resources when they are no longer needed.