Fix some bugs in xds flow control #2321

Job	Run time
download midwares and cache	15s
set-execution-conditions	9s
build agent and cache	1m 55s
Test for spring (1.5.0.RELEASE, Edgware.SR2, 1.5.0.RELEASE)	15m 27s
Test for spring (2.0.2.RELEASE, Finchley.RELEASE, 4.1.3, 2.0.0.RELEASE, 1.0.1)	15m 45s
Test for spring (2.1.0.RELEASE, Greenwich.RELEASE, 4.2.6, 2.1.0.RELEASE, 1.0.1)	15m 21s
Test for spring (2.2.0.RELEASE, Hoxton.RELEASE, 4.3.6, 2.2.0.RELEASE, 1.0.3)	15m 12s
Test for spring (2.3.0.RELEASE, Hoxton.RELEASE, 4.4.1, 2.2.8.RELEASE, 1.1.2)	15m 30s
Test for spring (2.4.0, 2020.0.0, 4.5.6, 2021.0.1.0, 1.1.4)	14m 58s
Test for spring (2.6.2, 2021.0.0, 4.5.10, 2021.1, 1.1.10)	14m 24s
Test for spring (2.7.2, 2021.0.3, 4.5.13, 1.1.12)	14m 7s
	2h 3m 3s

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