dout: add macros for libfmt-style logging

dout: add macros for libfmt-style logging

new dout macros use fmt::print() to write output directly to the
underlying ostream. this enables libfmt's printf-style formatting syntax
for ceph log output, without the string allocation overhead of:

    dout(10) << fmt::format("str={} count={}", str, count) << dendl;

which becomes:

    dout_fmt(10, "str={} count={}", str, count);

Signed-off-by: Casey Bodley <cbodley@redhat.com>