Move walredo process code under pgxn in the main 'neon' repository.

- Refactor the way the WalProposerMain function is called when started
  with --sync-safekeepers. The postgres binary now explicitly loads
  the 'neon.so' library and calls the WalProposerMain in it. This is
  simpler than the global function callback "hook" we previously used.

- Move the WAL redo process code to a new library, neon_walredo.so,
  and use the same mechanism as for --sync-safekeepers to call the
  WalRedoMain function, when launched with --walredo argument.

- Also move the seccomp code to neon_walredo.so library. I kept the
  configure check in the postgres side for now, though.

docs/sourcetree.md

@@ -52,6 +52,10 @@ PostgreSQL extension that implements storage manager API and network communicati
 
 PostgreSQL extension that contains functions needed for testing and debugging.
 
+`/pgxn/neon_walredo`:
+
+Library to run Postgres as a "WAL redo process" in the pageserver.
+
 `/safekeeper`:
 
 The neon WAL service that receives WAL from a primary compute nodes and streams it to the pageserver.