RFC : channel(), tspace() and kvspace()

[amitmurthy]

This PR relies on multiple dispatch on the container of the remote object referenced to by a RemoteRef to implement channels, tspaces and kvspaces.

In that sense it is an alternative approach to introducing channels and the like in Julia as compared to #5757

This PR introduces 3 new methods

channel() which returns a RemoteRef to a channel
tspace() which returns a RemoteRef to a tspace, i.e., a tuple space
kvspace() which returns a RemoteRef to a kvspace, i.e., a dict

tspace and kvspace are inspired by http://en.wikipedia.org/wiki/Linda_(coordination_language)

A channel can have a type and length associated with it. put blocks only when full, take only when empty. It is a queue.

A tspace can store tuples, where the first element is a key.
take(rr, key) will block till an element matching key becomes available in the tspace.
The key can be a Regex object, in which case, it blocks till a match is found.
The space itself is a queue so that any match is ordered as a FIFO.
fetch, isready, take all accept a key parameter and the entire tuple is returned.

A kvspace is like a dict, only exact matches are allowed in take and any duplicates in put results in an overwrite.
put is of the form put(rr, key, value).
fetch, isready, take all accept a key parameter and the associated value is returned.

[amitmurthy]

Closed in favor of #5817