Introduce an interface `ReadOnlyMemTable` for immutable memtables #3219

Job	Run time
build-linux-cmake-with-folly	27m 24s
build-examples	10m 52s
build-linux	17m 8s
build-linux-cmake-mingw	12m 39s
build-linux-cmake-with-benchmark	19m 1s
check-format-and-targets	26s
build-linux-cmake-with-folly-coroutines	31m 21s
build-linux-cmake-with-folly-lite-no-test	7m 51s
build-linux-make-with-folly	27m 58s
build-linux-make-with-folly-lite-no-test	9m 7s
build-linux-release	14m 1s
build-linux-release-rtti	12m 12s
build-fuzzers	8m 24s
build-linux-clang10-asan	16m 56s
build-linux-clang10-clang-analyze	17m 12s
build-linux-clang10-ubsan	16m 39s
build-linux-clang-13-no_test_run	7m 8s
build-linux-clang-no_test_run	11m 44s
build-linux-encrypted_env-no_compression	16m 6s
build-linux-gcc-10-cxx20-no_test_run	9m 22s
build-linux-gcc-11-no_test_run	9m 53s
build-linux-gcc-8-no_test_run	8m 32s
build-linux-mini-crashtest	28m 57s
build-linux-unity-and-headers	12m 47s
build-linux-clang10-mini-tsan	23m 55s
build-linux-java	15m 7s
build-linux-java-pmd	11m 41s
build-linux-java-static	11m 35s
build-macos	33m 28s
build-linux-static_lib-alt_namespace-status_checked	18m 53s
build-macos-cmake (true)	47m 33s
build-macos-cmake (false)	46m 20s
build-macos-java	25m 19s
build-macos-java-static-universal	20m 56s
build-macos-java-static	22m 24s
build-windows-vs2019	52m 34s
build-linux-arm	14m 30s
	11h 37m 55s

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