Speed-the-Plow By David Mamet Presented By Bad Habit Productions 8:00 pm tonight and Saturday, 2:00 pm Sunday Factory Theatre, 791 Tremont St., Boston. $12. Tickets and information They are the largely thankless theatrical roles, the parts that move a piece along but don't bring with them the promise of glory. Take Hamlet in Tom Stoppard's "Rosencrantz & Guilderstern Are Dead" and Rose Maxson in August Wilson's "Fences" - both necessary characters, but parts designed as......