Flush qlog streamer before closing the connection #4622

Job	Run time
quiche_windows (x86_64-pc-windows-msvc)	2m 53s
http3_test	1m 10s
quiche_windows (i686-pc-windows-msvc)	4m 0s
quiche	3m 43s
quiche_windows (x86_64-pc-windows-gnu)	4m 46s
quiche_macos	2m 46s
quiche (boringssl-boring-crate)	4m 30s
quiche_windows (i686-pc-windows-gnu)	5m 57s
quiche_ios (x86_64-apple-ios)	1m 50s
android_ndk_lts (aarch64-linux-android)	1m 32s
docker	3m 10s
quiche_ios (aarch64-apple-ios)	1m 44s
android_ndk_lts (armv7-linux-androideabi)	1m 30s
android_ndk_lts (x86_64-linux-android)	1m 33s
android_ndk_lts (i686-linux-android)	1m 22s
nginx (1.16.1)	1m 1s
quiche_multiarch (aarch64-unknown-linux-gnu)	3m 30s
quiche_multiarch (armv7-unknown-linux-gnueabihf)	3m 36s
quiche_multiarch (i686-unknown-linux-gnu)	2m 11s
	52m 44s

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