stream: split RecvBuf and SendBuf into submodules #10

Job	Run time
docker	3m 50s
android_ndk_lts (aarch64-linux-android)	2m 16s
android_ndk_lts (armv7-linux-androideabi)	2m 15s
http3_test	1m 12s
android_ndk_lts (x86_64-linux-android)	2m 24s
nginx (1.16.1)	1m 16s
android_ndk_lts (i686-linux-android)	2m 10s
quiche	3m 55s
quiche_ios (x86_64-apple-ios)	1m 19s
quiche_macos	1m 47s
quiche_ios (aarch64-apple-ios)	1m 8s
quiche_multiarch (aarch64-unknown-linux-gnu)	4m 53s
quiche_windows (x86_64-pc-windows-msvc)	3m 45s
quiche_windows (i686-pc-windows-msvc)	4m 30s
quiche_multiarch (armv7-unknown-linux-gnueabihf)	5m 52s
quiche_windows (x86_64-pc-windows-gnu)	3m 59s
quiche_multiarch (i686-unknown-linux-gnu)	3m 31s
quiche_windows (i686-pc-windows-gnu)	4m 24s
	54m 26s

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