Maine Shrimp Chowder with Crispy Pancetta and Chives

Name: Maine Shrimp Chowder with Crispy Pancetta and Chives
Ingredients: 1 pound Maine shrimp, peeled, shells (and heads, if you have them) reserved; 5 cups water; Kosher or fine sea salt; 1/2 teaspoon double-concentrated tomato paste (from a tube); 1 tablespoon unsalted butter; 1 ounce pancetta; 1 medium leek, white and pale green parts only, cut into very thin half-moons; 1/4 cup fennel bulb, finely diced; 1/4 cup celery, thinly sliced; 2 cups waxy potato, like Nicola or Yukon Gold, cut into ½ inch cubes; 1/2 cup half and half; freshly cracked pepper; snipped fresh chives for garnish