If we start walreceiver with identify_system.xlogpos() we will have race condition with
postgres start: postgres may request page that was modified with lsn
smaller than identify_system.xlogpos().
Current procedure for starting postgres will anyway be changed to something
different like having 'initdb' method on a pageserver (or importing some shared
empty database snapshot), so for now I just put start of first segment which
seems to be a valid record and is strictly before first lsn records.
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.
