Decouple HostBuffers from Stream

To allow asynchronous monitoring and output writing, the HostBuffers must be decoupled from Stream and handled separately.

• A HostBuffersManager should be created that will allocate a configurable number of HostBuffers.
• StreamWrapper and Stream should be adapted to be passed an HostBuffersManager const* on construction, from which they will request a HostBuffers* by index. The index will be passed in from the main loop.
• The main keeps track of what HostBuffers instances are free and hands them out together with slice indices whenever a slice is sent for processing