Fix some bugs in xds flow control #2376

Job	Run time
download midwares and cache	6s
set-execution-conditions	10s
build agent and cache	1m 34s
Test for spring (1.5.0.RELEASE, Edgware.SR2, 1.5.0.RELEASE)	15m 31s
Test for spring (2.0.2.RELEASE, Finchley.RELEASE, 4.1.3, 2.0.0.RELEASE, 1.0.1)	16m 5s
Test for spring (2.1.0.RELEASE, Greenwich.RELEASE, 4.2.6, 2.1.0.RELEASE, 1.0.1)	15m 1s
Test for spring (2.2.0.RELEASE, Hoxton.RELEASE, 4.3.6, 2.2.0.RELEASE, 1.0.3)	14m 45s
Test for spring (2.3.0.RELEASE, Hoxton.RELEASE, 4.4.1, 2.2.8.RELEASE, 1.1.2)	15m 12s
Test for spring (2.4.0, 2020.0.0, 4.5.6, 2021.0.1.0, 1.1.4)	14m 52s
Test for spring (2.6.2, 2021.0.0, 4.5.10, 2021.1, 1.1.10)	14m 26s
Test for spring (2.7.2, 2021.0.3, 4.5.13, 1.1.12)	14m 19s
	2h 2m 1s

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