Async http client

[dkargin]

Implementing better HttpClient for rosmaster:

1. Async operation
2. Can track status of each request directly.
3. Will be used for sending requests from rosmaster to other nodes or masters.
4. Will replace more parts from xmlrpc.

[dkargin]

Additional thread at miniros::RPCManager was removed. Now all XMPRPC requests are spinned by PollSet instance from PollManager, or processed later by internal callback queue.

All usage of XmlRpc::XmlRpcClient in miniros::RPCManager and miniros::MasterLink was replaced by miniros::http::HttpClient. It has resolved some potential internal racing conditions and deadlocks and reduced usage of xmlrpc library overall.

Additional functionality to PollSet was added:

1. Can set timeout event for socket. This event is not very precise (about 50ms resolution) but sufficient for reconnection logic.
2. Sockets with zero event flags will be removed from epoll, but still be registered inside PollSet. It will reduce number of fired events for objects in some idle or semi-disconnected state.