Update FilePrefetchBuffer::Read to reuse file system buffer when possible #3340

Job	Run time
build-linux	17m 4s
build-linux-cmake-with-folly	27m 43s
build-linux-cmake-with-benchmark	18m 57s
build-linux-cmake-mingw	12m 5s
build-linux-cmake-with-folly-coroutines	31m 56s
build-linux-make-with-folly	28m 53s
build-linux-make-with-folly-lite-no-test	9m 20s
build-fuzzers	8m 28s
build-linux-clang-13-no_test_run	7m 15s
build-linux-cmake-with-folly-lite-no-test	7m 48s
build-linux-clang-no_test_run	11m 45s
build-linux-encrypted_env-no_compression	15m 53s
build-linux-release	14m 2s
build-linux-release-rtti	12m 19s
check-format-and-targets	24s
build-examples	10m 32s
build-linux-arm	14m 27s
build-linux-clang10-asan	16m 49s
build-linux-clang10-clang-analyze	17m 8s
build-linux-clang10-mini-tsan	24m 3s
build-linux-clang10-ubsan	16m 30s
build-linux-gcc-10-cxx20-no_test_run	9m 29s
build-linux-gcc-11-no_test_run	10m 9s
build-linux-gcc-8-no_test_run	8m 39s
build-linux-java	14m 56s
build-linux-java-pmd	11m 55s
build-linux-java-static	13m 8s
build-linux-mini-crashtest	48m 12s
build-linux-static_lib-alt_namespace-status_checked	18m 54s
build-linux-unity-and-headers	12m 52s
build-macos	17m 11s
build-macos-cmake (true)	41m 9s
build-macos-java	16m 59s
build-macos-java-static	27m 2s
build-macos-cmake (false)	44m 46s
build-macos-java-static-universal	21m 18s
build-windows-vs2019	54m 48s
	11h 34m 48s

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