Report timespans for promotion and prewarm (#12730)

- Return sub-actions time spans for prewarm, prewarm offload, and
promotion in http handlers.
- Set `synchronous_standby_names=walproposer` for promoted endpoints.
Otherwise, walproposer on promoted standby ignores reply from safekeeper
and is stuck on lsn COMMIT eternally.

test_runner/regress/test_replica_promotes.py

@@ -145,6 +145,7 @@ def test_replica_promote(neon_simple_env: NeonEnv, method: PromoteMethod):
         stop_and_check_lsn(secondary, None)
 
     if method == PromoteMethod.COMPUTE_CTL:
+        log.info("Restarting primary to check new config")
         secondary.stop()
         # In production, compute ultimately receives new compute spec from cplane.
         secondary.respec(mode="Primary")