This is a first pass on introducing an experimental "channel" concept that can be used for direct worker-worker communication, bypassing the usual Ray Core components like the driver and raylet.

Channels are implemented as mutable plasma objects. The object can be written multiple times by a client. The writer must specify the number of reads that can be made before the written object value is no longer valid. Reads block until the specified version or a later one is available. Writes block until all readers are available. Synchronization between a single writer and multiple readers is performed through a new header for plasma objects that is stored in shared memory.

API:

    channel: Channel = ray.experimental.channel.Channel(buf_size): Client uses the normal ray.put path to create a mutable plasma object. Once created and sealed for the first time, the plasma store synchronously reads and releases the object. At this point, the object may be written by the original client and read by others.
    channel.write(val): Use the handle returned by the above to send a value through the channel. The caller waits until all readers of the previous version have released the object, then writes a new version.
    val = channel.begin_read(): Blocks until a value is available. Equivalent to ray.get. This is the beginning of the client's read.
    channel.end_read(): End the client's read, marking the channel as available to write again.

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Signed-off-by: Stephanie Wang <swang@cs.berkeley.edu>