Implements the LPOS command for #2055

Had to add a new Message type to accommodate.

@NickCraver, one thing that might be slightly controversial, I set rank to 1 and maxlen to 0 by default, this is operationally equivalent to not adding them at all but very much simplifies the message creation without having to do any other array/list allocations. LMK what you think of this.

Also is there a better way to extract an array of expected Value-types? maybe a new ResultsProcessor?