Removes connection_base class. #558

Job	Run time
B2 gcc-11	5m 7s
B2 msvc-14.2	8m 13s
B2 clang-14	5m 37s
CMake msvc-14.2 Debug C++17	7m 0s
B2 msvc-14.3	9m 30s
CMake gcc-11 17 Debug	7m 18s
CMake msvc-14.2 Release C++17	7m 52s
CMake gcc-11 20 Release	9m 58s
CMake msvc-14.3 Debug C++20	6m 40s
CMake clang-11 17 Debug	6m 0s
CMake msvc-14.3 Release C++20	6m 15s
CMake clang-11 20 Debug	6m 33s
CMake clang-13 17 Release	9m 2s
CMake clang-13 20 Release	9m 47s
CMake clang-14 17 Debug -stdlib=libc++	7m 30s
CMake clang-14 20 Release -stdlib=libc++	10m 35s
	2h 2m 57s

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