Support several postgres instances on top of a single pageserver.

Each postgres will use its own page cache with associated data
structures. Postgres system_id is used to distinguish instances.
That also means that backup should have valid system_id stashed
somewhere. For now I put '42' as sys_id during S3 restore, but
that ought to be fixed.

Also this commit introduces new way of starting WAL receivers:
postgres can initiate such connection by calling 'callmemaybe $url'
command in the page_service -- that will start appropriate wal-redo
and wal-receiver threads. This way page server may start without
a priori knowledge of compute node addreses.

.github/workflows/testing.yml

@@ -27,4 +27,4 @@ jobs:
 
       - name: Run test
         run: |
-          cargo test --test test_pageserver -- --nocapture --test-threads=1
+          cargo test --test test_pageserver -- --nocapture --test-threads=1