check the return value of inet_ntop()

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: check the return value of epoch_log_read_with_timestamp() correctly

log->nr_nodes is an unsigned integer, so we cannot use it for
assigning the negative value.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
tests: add support for testing direct IO mode

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
compile with -DNDEBUG by default

assert() is used to confirm code's correctness and it's no problem to
remove it for a release build.

This also replaces assert() in cluster/local.c with if/panic() because
it abuses assert() for error checking.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>