CA-418227: Detect rolling upgrade after setting software_version

The `host.software_version` field is updated in dbsync_slave, and only
after that has happened, the coordinator can detect whether rolling pool
upgrade is in progress (by comparing versions across the pool from this
DB field).

Unfortunately, the `detect_rolling_upgrade` function is first called
just before dbsync_slave runs, as part of `Db_gc.single_pass`, right
after starting the DB engine on the coordinator. This will eventually
get rectified once the DB thread is running, but unfortunately a
function in dbsync_master (which sets the `host.allowed_operations`
fields for all hosts) relies on the RPU state to be correct. Therefore,
force the RPU detection immediately after dbsync_slave.

This isn't a great solution... but any change to the startup sequence
seems to break _something_ and this looks relatively safe.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>

Author: robhoes

ocaml/xapi/dbsync_master.ml

@@ -354,6 +354,7 @@ let update_pool_recommendations_noexn ~__context =
 (* Update the database to reflect current state. Called for both start of day and after
    an agent restart. *)
 let update_env __context =
+  Db_gc.detect_rolling_upgrade ~__context ;
   debug "creating root user" ;
   Create_misc.create_root_user ~__context ;
   debug "creating pool record" ;