Remove Sync trait bounds on callback futures #605

Job	Run time
clippy	1m 50s
Cross build from macOS to Linux (x86_64-unknown-linux-gnu)	3m 12s
Cross build from macOS to Linux (FIPS) (x86_64-unknown-linux-gnu)	5m 52s
rustfmt	7s
Test features	5m 21s
Test FIPS integration	2m 56s
Test (stable)	3m 58s
Test (macos-x86_64)	5m 35s
Test (arm-android)	3m 12s
Test (arm64-android)	3m 46s
Test (i686-android)	3m 22s
Test (x86_64-android)	2m 59s
Test (aarch64-ios)	4m 10s
Test (aarch64-ios-sim)	5m 1s
Test (x86_64-ios)	3m 48s
Test (i686-linux)	2m 57s
Test (arm-linux)	3m 4s
Test (aarch64-linux)	4m 23s
Test (x86_64-musl)	4m 11s
Test (x86_64-mingw)	3m 11s
Test (i686-msvc)	7m 9s
Test (x86_64-msvc)	11m 56s
	1h 32m 0s

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