Removing external dependency on boost::signals2 #5261

I am proposing we phase out boost::signals2 in favour of the small/fast implementation contained in this commit. I ran the testsuite, there are no errors. I ran run time tests, no regressions. This does not touch consensus critical code.

https://github.com/pbhogan/Signals Extremely efficient (based on delegates that generate only two lines of ASM code). No inheritance required. No third-party preprocessor required. Works seamlessly with global functions, object methods, virtual methods and static class methods. Very portable. This should work on any reasonable C++ compiler. Really simple to drop in and use. Implemented fully in templates.

also fixed a bug where a .disconnect was missing in GUI code.

Do you have any indication boost::signals is a bottleneck anywhere? Don't fix what is not broken.

Also: this doesn't seem to have thread-safe connect/disconnect, unlike boost.

@sinetek I'd suggest you split the .disconnect bugfix out of this pull request.

I agree in principle that I shan't fix what isn't broken. This seemed a good idea at the time. I still don't like boost that much though. I'll split the bugfix out and submit.

As much as i dislike boost, agreed on not changing it without good reason. However, this might be worth revisiting after a move to c++11, since that opens the door to several other lightweight implementations.

I'm completely sick of this "I don't like boost". Not liking boost doesn't mean that any random library or hand-rolled alternative is better. We also don't want to maintain every single thing as part of Bitcoin Core. There's a reason that utility libraries exist. Closing.