SOUTH YARMOUTH — One by one, nearly 2,000 police officers marched solemnly by the casket of their slain brother-in-blue Sean Gannon, whose body will remain peacefully in St. Pius X Church under the close guard of fellow officers until his funeral this morning.Police paraded single-file into the church to the wail and drone of bagpipes, with hundreds of others standing by, waiting to pay their respects to the 32-year-old officer killed delivering a warrant to a 29-year-old man in Barnstable last week.