gossip: Write to `contact-info.bin` file with a `BufWriter` (#4115) #8180

Job	Run time
android (ubuntu-20.04, x86_64-linux-android)	2m 19s
android (ubuntu-20.04, aarch64-linux-android)	2m 20s
ios (macos-13, aarch64-apple-ios)	4m 14s
android (ubuntu-20.04, i686-linux-android)	2m 20s
android (ubuntu-20.04, armv7-linux-androideabi)	2m 34s
ios (macos-13, x86_64-apple-ios)	2m 36s
ios (macos-13, aarch64-apple-darwin)	3m 56s
ios (macos-13, x86_64-apple-darwin)	4m 26s
error_reporting	0s
	24m 45s

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