Add hidden CLI arg to control number of shred sigverify threads (#3046) #5453

Job	Run time
android (ubuntu-20.04, x86_64-linux-android)	2m 45s
ios (macos-12, aarch64-apple-ios)	4m 2s
ios (macos-12, x86_64-apple-ios)	3m 35s
ios (macos-12, aarch64-apple-darwin)	3m 52s
ios (macos-12, x86_64-apple-darwin)	4m 32s
android (ubuntu-20.04, aarch64-linux-android)	3m 0s
android (ubuntu-20.04, i686-linux-android)	2m 49s
android (ubuntu-20.04, armv7-linux-androideabi)	3m 3s
error_reporting	0s
	27m 38s

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