Add binary support to ClusterShell workers

[thiell]

Currently ClusterShell only supports reading lines of text. Remove this limitation and add support for binary read/writes.

[thiell]

[st-cea] Need more work for a clean implementation (add send queue).

[thiell]

[st-cea] Binary write is already implemented. Additionally, as of r383, due (thanks) to the use of EnginePort in order to send any messages to remote worker, binary support is automagically available in clush (for stdin forwarding to remote nodes).

This ticket now tracks more in-depth modifications of the library API to avoid relying on lines, for example, when reading a buffer in a worker.