main: destroy main fiber and whole cord on return

main() used to skip most of modules destruction in
tarantool_free(). That got ASAN complaining on clang-13 about a
leak of a fiber on_stop trigger which was allocated in Lua.

The patch makes fiber_free() called for the main cord. It destroys
and frees all the fibers together with their on_stop triggers.

Closes #7259

NO_CHANGELOG=Not a visible change
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