improve timestamp type: int -> uint. #3655

Job	Run time
multiple-arch-aarch64	22m 44s
multiple-arch-armv7	21m 47s
cygwin64-cache	1m 57s
utest-regression-blackbox-test	9m 30s
fast	1s
cygwin64	14m 57s
build-centos7	4m 55s
build-cross-aarch64	5m 2s
build-ubuntu16	2m 59s
build-ubuntu18	3m 5s
build-cross-arm	5m 9s
build-ubuntu20	3m 28s
coverage	3m 30s
multiple-arch-amd64	2m 26s
test-done	1s
	1h 41m 31s

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