Another preparatory commit for pg15 support:
* generate bindings for both pg14 and pg15;
* update Makefile and CI scripts: now neon build depends on both PostgreSQL versions;
* some code refactoring to decrease version-specific dependencies.
Re-export only things that are used by other modules.
In the future, I'm imagining that we run bindgen twice, for Postgres
v14 and v15. The two sets of bindings would go into separate
'bindings_v14' and 'bindings_v15' modules.
Rearrange postgres_ffi modules.
Move function, to avoid Postgres version dependency in timelines.rs
Move function to generate a logical-message WAL record to postgres_ffi.
Make it remember when timeline starts in general and on this safekeeper in
particular (the point might be later on new safekeeper replacing failed one).
Bumps control file and walproposer protocol versions.
While protocol is bumped, also add safekeeper node id to
AcceptorProposerGreeting.
ref #1561
