wal_storage.rs was split up from timeline.rs, safekeeper.rs and send_wal.rs,
and now contains all WAL related code from the safekeeper. Now there are
PhysicalStorage for persisting WAL to disk and WalReader for reading it.
This allows optimizing PhysicalStorage without affecting too much of other
code.
Also there is a separate structure for persisting control file now in
control_file.rs.
Currently ztimelineids are unique, but all APIs accept the pair, so let's keep
it everywhere for uniformity.
Carry around ZTTId containing both ZTenantId and ZTimelineId for simplicity.
(existing clusters on staging ought to be preprocessed for that)
This patch includes attach/detach http endpoints in pageservers. Some
changes in callmemaybe handling inside safekeeper and an integrational
test to check migration with and without load. There are still some
rough edges that will be addressed in follow up patches
Do it separately with SafekeeperPostgresCommand enum as a result. Since query is
always C string, switch postgres_backend process_query argument from Bytes to
&str.
Make passing ztli/ztenant id in safekeeper connection string optional; this is
needed for upcoming intra-safekeeper heartbeat cmd which is not bound to any
timeline.
