encoding: add round_trip_encode()/decode() for chrono types

encoding: add round_trip_encode()/decode() for chrono types

the default encodings for chrono types were made to be
backward-compatible with utime_t, so truncated seconds to 32 bits

adds new functions that encode these chrono types using their underlying
representation, which for ceph::real_time and ceph::timespan is
'nanoseconds as uint64_t'

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