Fix build errors and warnings from #184

[JJL772]

Fixes a couple of oopses in #184. I mistakenly used C++11 constructs (nullptr, std::shared_ptr) which breaks older compilers.

There was also a warning due to linkage type mismatch in the BeaconHandler, so I've moved BeaconCleanupHandler to a non-anonymous detail namespace

[anjohnson]

Thanks. I have been testing some updates to the CI configurations that fix the Win2019 builds, and also clear up some build warnings on Clang. They are failing when built with MSC and the C++17 standard (new build config), so I'll see if this PR resolves those.

[AppVeyorBot]

❌ Build pvAccessCPP 1.0.141 failed (commit b011e86af5 by @JJL772)

[JJL772]

both of these issues were introduced by my PR #191, it's unrelated to those preexisting MSVC build failures

[AppVeyorBot]

❌ Build pvAccessCPP 1.0.141 failed (commit 4a64c530b3 by @JJL772)

[AppVeyorBot]

❌ Build pvAccessCPP 1.0.141 failed (commit 4a64c530b3 by @JJL772)

[anjohnson]

When you see an Appveyor build failure that takes exactly 1hr to fail, those are (usually) known issues where one or two of the tests can hang, resulting in the CI job timing out. That's why you see the Appveyor Bot reporting multiple failures above, I'm rerunning the failed jobs so that they should eventually all pass (as long as they're caused by those hanging tests). Unfortunately re-triggering failed jobs on Appveyor causes the "failed" link in the PR comment to stop working, but eventually they should pass and the link will show a green check-mark.

My point of wanting to test with my CI changes is for the GitHub Win2019 jobs that aren't being run; they will become "7.0 Win-22" builds instead.

[JJL772]

Ah, I misunderstood your previous comment. Feel free to test away