Now most of CI check time is spent during dependencies installation and compilation (~ 10min total). Use actions/cache@v2 to cache things between checks. This commit sets up two caching targets:
* ./tmp_install with postgres build files and installed binaries uses $runner.os-pg-$pg_submodule_revision as a cache key and will be rebuilt only if linked submodule revision changes.
* ./target with cargo dependencies. That one uses hash(Cargo.lock) as a caching key and will be rebuilt only on deps update.
Also add tg notifications in a passing.
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.
