fix race condition when cancelling requests after becoming a seed #3268

Job	Run time
check headers	3m 56s
build (asio-debugging=on picker-debugging=on)	29m 18s
clang-tidy	9m 54s
build (extensions=off logging=off streaming=off super-seeding=off share-mode=off predictive-piece...	22m 31s
build (crypto=gcrypt)	25m 17s
Fuzzers	8m 27s
build (mmap-disk-io=off)	28m 32s
pre-commit	46s
Simulations	19m 1s
Tests (address-sanitizer=norecover undefined-sanitizer=norecover crypto=openssl)	16m 19s
Tests (toolset=clang logging=off address-sanitizer=norecover undefined-sanitizer=norecover)	11m 22s
build dist (ubuntu-20.04)	17m 15s
Tests (thread-sanitizer=norecover crypto=openssl release debug-symbols=on cxxflags=-Wno-tsan)	17m 57s
build dist (ubuntu-22.04)	15m 53s
Tests (crypto=gnutls)	12m 9s
Tests (deprecated-functions=off)	11m 0s
	4h 9m 37s

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