Dispatch mode for method_lookup Subrequests
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There is a pattern in the wild of using OpenSRF's method_lookup() facility
to decide between one of several local methods when delegating to pre-existing
logic.  Often times, we want to simply hand control over to another method,
but the output of a subrequest's run() is an array of results.  The caller has
to know if, and how, to restructure the result for the client.

Instead, we can now call dispatch() instead of run() and have OpenSRF session
control completely passed to the delegate code.  This way, the delegate code
need not know anything about its caller, and vice versa.