Terminate background worker if postmaster dies and build with bindgen.

[bizwark]

This change calls PostmasterIsAliveInternal so that the background worker for these extensions will exit if the postmaster received a SIGKILL signal. This change also includes build changes to verify that the signature of PostmasterIsAliveInternal as called from lib.rs matches what is in the Postgres specified by pg_config, so that if the function signature is ever changed in the future we will fail to build rather than crashing at runtime.

[hlinnaka]

Is this necessary? If the signature of PostmasterIsAliveInternal changes, wouldn't you get a compilation error even without this?

[hlinnaka]

Now that I think about this some more, I think using wait_latch() might be the most straightforward solution. That already exists in pgrx. We don't need to wait on any latch here, but you can use wait_latch() just to sleep. I think you can replace the sleep with wait_latch(), and it will get woken up if the postmaster dies. Or if you use the WL_EXIT_ON_PM_DEATH, it will directly exit the process, which is what we want here I believe.

Sorry I didn't realize that before, I somehow thought that the loop needs to wait on something else at the same time. I didn't realize you could use wait_latch() just to check for postmaster death and/or to replace the sleep.