SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

So far in all our uses of ScopeGuard, the type of the callable is
usually just a lambda or a function pointer, so there is no need
to rely on std::function's type erasure.

While the cost of using std::function is probably negligible, it still
causes some unnecessary overhead that can be avoided by making
ScopeGuard a templated class. Thanks to class template argument
deduction in C++17 most existing usages do not even need to be changed.

See https://godbolt.org/z/KcoPni for a comparison between
a ScopeGuard that uses std::function and one that doesn't

g_compressAndDumpStateSyncEvent was Set() before destruction of file object (i.e. before flushing changes and closing file).

Also, adds Common::ScopeGuard wrapper for RAII.