Place the bones in a pot and cover with 2 quarts water. Add the top halves of the lemon grass, the lime leaves, and the shells of the shrimp. Simmer for an hour or so, skimming off the foam, until the water is reduced by nearly half. Strain the stock. Mince the lower portion of the lemon grass and the chiles. Add to the pot with the paste and fish sauce and simmer for another 15 minutes. Add the mushrooms, halved, and peeled shrimp and cook several minutes until the shrimp are done. Add the scallions, lime juice, and cilantro and serve hot.