[RF] Make RooRealIntegral conform with the client-server interface

[guitargeek]

In pass through mode, the RooRealIntegral should have registered the function as a value server, because we directly depend on its value.

It's important to do this correctly, because the new BatchMode uses the value server interface to analyze the computation graph.

Furthermore, it is now ensured that in pass-through mode, no servers are registered other than the actual function and the additional factorized observables.

There are also some other commits to ensure no invalid proxies are created via copy/move assignment and construction.

Final commit in this PR does some code modernization of the RooRealIntegral.

[phsft-bot]

Build failed on mac1015/cxx17. Running on macitois21.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.roofit.roofitcore.test.gtest_roofit_roofitcore_test_testRooWrapperPdf
• projectroot.roottest.root.roofitstats.vectorisedPDFs.gtesttest_root_roofitstats_vectorisedPDFs_testNestedPDFs

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Build failed on windows10/cxx14. Running on null:C:\build\workspace\root-pullrequests-build See console output.

Errors:

• [2022-08-10T22:38:24.180Z] C:\build\workspace\root-pullrequests-build\root\interpreter\llvm\src\lib\CodeGen\SpillPlacement.cpp(383,1): fatal error C1060: compiler is out of heap space [C:\build\workspace\root-pullrequests-build\build\interpreter\llvm\src\lib\CodeGen\LLVMCodeGen.vcxproj]
• [2022-08-10T22:38:24.561Z] C:\build\workspace\root-pullrequests-build\root\interpreter\llvm\src\tools\clang\lib\AST\APValue.cpp(816,1): fatal error C1060: compiler is out of heap space [C:\build\workspace\root-pullrequests-build\build\interpreter\llvm\src\tools\clang\lib\AST\obj.clangAST.vcxproj]
• [2022-08-11T02:09:30.215Z] MSBUILD : error MSB4166: Child node "11" exited prematurely. Shutting down. Diagnostic information may be found in files in "C:\Users\sftnight\AppData\Local\Temp" and will be named MSBuild_*.failure.txt. This location can be changed by setting the MSBUILDDEBUGPATH environment variable to a different directory.