Add global-state request to server mode

[sim642]

Builds on #1012.

This request allows getting the entire global state (not too practical), but also filtering by constraint variable queries (e.g. by variable vid or node). The latter could be useful for displaying relevant global invariant states in the abstract debugger, relating to the current node or global variables in multi-threaded mode.

[FeldrinH]

I currently don't see much use for this in my use case. arg/eval and cil/varinfos together supersede most of the functionality this would have provided for abstract debugging i.e. finding and showing values of global variables.

[sim642]

Even if this has no immediate use, I'm merging this because it's a very localized addition and includes a bunch of name improvements that are useful during other debugging as well.