libhdfs++: Prevent Requests from holding dangling pointer to RpcEngine

The lifetime of Request objects is tied to the worker thread(s) in the async event loop. In the current code there's nothing that prevents a request from outliving the RpcEngine (bound to FileSystem) while it's waiting for IO. If the Request, or a task that makes a new request, outlives the RpcEngine it attempts to dereference a dangling pointer and either crashes or continues to run with bad data.

Proposed fix is to reference count the RpcEngine via shared_ptr so that Requests can hold a weak_ptr to it. When a request or RpcConnection attempting to make a request needs something from the RpcEngine like a call id number it can promote the weak_ptr to a shared_ptr. If it's unable to promote because the RpcEngine has been destroyed the Request's handler can be invoked with an appropriate error message. A weak_ptr must be used rather than a shared_ptr to avoid reference cycles.