[io] Extract tkey walk logic from TFile::Map() #15717

Job	Run time
Windows 10 x64 Release	27m 34s
mac13 ARM64 LLVM_ENABLE_ASSERTIONS=On, builtin_zlib=ON	31m 11s
Windows 10 x86 Release	43m 9s
mac14 X64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	1s
mac15 ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	35m 58s
mac-beta ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	47m 24s
fedora40 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	54m 49s
Upload Event Payload	3s
fedora41 LLVM_ENABLE_ASSERTIONS=On	45m 27s
alma8 LLVM_ENABLE_ASSERTIONS=On	40m 2s
alma9 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 5m 45s
ubuntu22 imt=Off, LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 5m 45s
ubuntu2404 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 5m 45s
ubuntu2410 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 5m 45s
debian125 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	1h 5m 45s
alma9 modules_off runtime_cxxmodules=Off	34m 13s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, fortran=OFF	38m 20s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo	51m 33s
alma9-clang clang LLVM_ENABLE_ASSERTIONS=On, CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	35m 47s
	13h 34m 16s

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