Use higher priority for PVF preparation in dispute/approval context (#4172)

Related to https://github.com/paritytech/polkadot-sdk/issues/4126
discussion

Currently all preparations have same priority and this is not ideal in
all cases. This change should improve the finality time in the context
of on-demand parachains and when `ExecutorParams` are updated on-chain
and a rebuild of all artifacts is required. The desired effect is to
speed up approval and dispute PVF executions which require preparation
and delay backing executions which require preparation.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/core/pvf/src/host.rs

@@ -197,7 +197,7 @@ impl Config {
 			prepare_worker_program_path,
 			prepare_worker_spawn_timeout: Duration::from_secs(3),
 			prepare_workers_soft_max_num: 1,
-			prepare_workers_hard_max_num: 1,
+			prepare_workers_hard_max_num: 2,
 
 			execute_worker_program_path,
 			execute_worker_spawn_timeout: Duration::from_secs(3),

polkadot/node/core/pvf/src/priority.rs

@@ -14,17 +14,18 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
-/// A priority assigned to execution of a PVF.
+/// A priority assigned to preparation of a PVF.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum Priority {
 	/// Normal priority for things that do not require immediate response, but still need to be
 	/// done pretty quick.
 	///
-	/// Approvals and disputes fall into this category.
+	/// Backing falls into this category.
 	Normal,
 	/// This priority is used for requests that are required to be processed as soon as possible.
 	///
-	/// For example, backing is on a critical path and requires execution as soon as possible.
+	/// Disputes and approvals are on a critical path and require execution as soon as
+	/// possible to not delay finality.
 	Critical,
 }