Add support for passing client_data_t to flush_cache #2827

Job	Run time
build (asio-debugging=on picker-debugging=on)	29m 23s
check headers	5m 57s
build (extensions=off logging=off streaming=off super-seeding=off share-mode=off predictive-piece...	46m 15s
Fuzzers	8m 39s
build (crypto=gcrypt)	26m 24s
pre-commit	44s
clang-tidy	13m 27s
build (mmap-disk-io=off)	28m 12s
build dist (ubuntu-20.04)	16m 34s
Simulations	34m 46s
build dist (ubuntu-22.04)	16m 1s
Tests (address-sanitizer=norecover undefined-sanitizer=norecover crypto=openssl)	32m 4s
Tests (toolset=clang logging=off address-sanitizer=norecover undefined-sanitizer=norecover)	11m 18s
Tests (thread-sanitizer=norecover crypto=openssl release debug-symbols=on cxxflags=-Wno-tsan)	17m 56s
Tests (crypto=gnutls)	12m 10s
Tests (deprecated-functions=off)	11m 17s
	5h 11m 7s

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