Split concepts of an input and a row stream

Currently there is a ssumed to be a 1:1 correspondence between an input and a row stream. E.g., Thor's input interface is derived from IRowStream.

Logically that should be split. I'm not 100% sure of the correct structure. I think we want the following:

• Activities can have multiple logical outputs and logical inputs.
• Each of those inputs and outputs could be processed in parallel with different row streams.
• Different inputs (and in the future outputs) could each have different numbers of streams. For instance, the RHS of a lookup join may have a single stream, but the LHS may have multiple streams.

(Note input isn't the most accurate name, I am going to use a Row channel in the following to see if it is clearer.)

Here is a straw man to shoot down and improve:

interface IActivity
    unsigned numOutputs();
    unsigned numInputs();
    unsigned numInputStrands(unsigned whichInput);
    IRiowChannel * queryOutput(unsigned n);
    void setInput(unsigned whichInput, IRowChannel * input);
    void setInputStrand(unsigned whichInput, unsigned whichStrand, IRowStreamEx * stream);

interface IRowChannel
    unsigned numStrands();
    IRowStream * queryStrand(unsigned n);
    IOutputMetaData * queryMetaData();

interface IRowStreamEx
    const void * nextRow();
    void stop(bool abort);
    ...

At this point the row channel interfaces will no longer be derived from the rowstream interface. I'm not sure if it is worth the effort of having a shared row channel in thor and roxie.

I think that matches our current activity structures relatively well. When connecting a particular output/input between activities, roxie should be able to automatically create the rowstream junctions to merge from 1->M M->1 etc. (It will need more information and logic...)

Note: Eventually the row probes will also potentially need to have multiple strand variants..