Store GUC variables in local process memory to avoid IPC shenanigans

[Medvecrab]

For more context and reasoning see issue #85

[mbanck-cd]

I have built Postgresql 17 pg_wait_sampling packages with this PR and tested it successfully (albeit not very thoroughly).

The usage of postmaster/interrupt.h means pg_wait_sampling will no longer compile on v12 (at least I got a build failure when I tried), but that one is EOL, so I guess it is ok?

gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -fno-omit-frame-pointer -g -O2 -ffile-prefix-map=/build/pg-wait-sampling-3SEHBq/pg-wait-sampling-1.1.6=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -I. -I./ -I/usr/include/postgresql/12/server -I/usr/include/postgresql/internal  -Wdate-time -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2   -c -o collector.o collector.c
collector.c:19:10: fatal error: postmaster/interrupt.h: No such file or directory
   19 | #include "postmaster/interrupt.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[1]: *** [<builtin>: collector.o] Error 1

[Medvecrab]

Yeah, I've seen CI fails, but since PostgreSQL 12 is no longer supported, I don't think we should worry too much. We dropped support for PostgreSQL 11 and older some time ago, so I think we will drop pg12 support some time later as well.

[mbanck-cd]

Yeah, I've seen CI fails, but since PostgreSQL 12 is no longer supported, I don't think we should worry too much. We dropped support for PostgreSQL 11 and older some time ago, so I think we will drop pg12 support some time later as well.

Right, in that case, README.md should be adjusted for s/12/13/ as minimum version, either in this PR or in a follow-up commit.

[Medvecrab]

I've rebased this branch onto actual master, fixed all review comments