internal: use a sync.Pool of msgpack.Decoder #4184

Job	Run time
run-tests-ce (1.20, 1.10, false, false)	12m 3s
run-tests-ce (1.20, 2.8, false, false)	12m 4s
testing_mac_os (1.20, macos-13, brew)	2m 22s
run-tests-ce (1.20, 2.10, false, false)	12m 42s
testing_mac_os (1.20, macos-13, 1.10.15)	13m 3s
run-tests-ce (1.20, master, false, false)	2m 13s
testing_mac_os (1.20, macos-14, brew)	1m 20s
run-tests-ce (stable, 1.10, false, false)	12m 0s
testing_mac_os (stable, macos-13, brew)	2m 1s
run-tests-ce (stable, 2.8, false, false)	12m 1s
testing_mac_os (stable, macos-13, 1.10.15)	12m 30s
run-tests-ce (stable, 2.10, false, false)	12m 35s
testing_mac_os (stable, macos-14, brew)	1m 13s
run-tests-ce (stable, master, false, false)	2m 1s
run-tests-ce (master, true, 1.20)	1m 57s
run-tests-ce (master, true, 1.20, false)	1m 51s
	1h 53m 56s

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