Place the chicken in a crockpot and add curry paste, curry powder, coconut milk, and broth. Cook on low for 6-8 hours. About 30 minutes before serving (around the time you're starting the rice), shred the chicken, then whisk the cornstarch together with the fish sauce and lime juice until smooth, and pour into crockpot. Add sugar, stir to combine, and let cook for 30 more minutes, until sauce is thickened (add more cornstarch if you want it even thicker).