msgpack: fix decoding intervals with int64

It is possible for interval to have days, hours, minutes and seconds
larger than INT_MAX (or less than INT_MIN). Before this patch, msgpack
decoding had failed to parse intervals with msgpack int64 and uint64.
int64_t should be enough to store any value allowed for datetime
intervals.

Closes #8887

NO_DOC=small bug fix

(cherry picked from commit 01c7ae11)