refactor so connection_write_to_buf() never fails

src/or/circuit.c

src/or/connection.c

src/or/connection_edge.c

src/or/connection_or.c

src/or/cpuworker.c

src/or/directory.c

src/or/dns.c

src/or/main.c

src/or/onion.c

src/or/or.h