Allow long running Exprs to report status to the system

[GoogleCodeExporter]

Long running functions can timeout during map/reduce when one record is
processed for a long time.  We should provide a way that any Expr can
report that progress is being made.  This probably involves placing the
hadoop reporter inside the jaql runtime context, but we will probably want
to abstract away from hadoop a little bit so it works in serial code as well.

This feature should consider a larger feature of reporting on the status of
a query, even when multiple map/reduce jobs are running concurrently.  It
should also include reporting of things besides progress, eg, number of
exceptions that were handled.

Original issue reported on code.google.com by [email protected] on 12 Mar 2009 at 8:26