Port Clock functions to use Duration class (#19229)

This changes the arguments in clock functions to be `Duration` and
converts call sites and constants into `Duration`. There are still some
more functions around that should be converted (e.g.
`timeout_deferred`), but we leave that to another PR.

We also changes `.as_secs()` to return a float, as the rounding broke
things subtly. The only reason to keep it (its the same as
`timedelta.total_seconds()`) is for symmetry with `as_millis()`.

Follows on from https://github.com/element-hq/synapse/pull/19223

tests/federation/transport/server/test__base.py

@@ -30,6 +30,7 @@ from synapse.http.server import JsonResource
 from synapse.server import HomeServer
 from synapse.types import JsonDict
 from synapse.util.cancellation import cancellable
+from synapse.util.duration import Duration
 from synapse.util.ratelimitutils import FederationRateLimiter
 
 from tests import unittest
@@ -53,13 +54,13 @@ class CancellableFederationServlet(BaseFederationServlet):
     async def on_GET(
         self, origin: str, content: None, query: dict[bytes, list[bytes]]
     ) -> tuple[int, JsonDict]:
-        await self.clock.sleep(1.0)
+        await self.clock.sleep(Duration(seconds=1))
         return HTTPStatus.OK, {"result": True}
 
     async def on_POST(
         self, origin: str, content: JsonDict, query: dict[bytes, list[bytes]]
     ) -> tuple[int, JsonDict]:
-        await self.clock.sleep(1.0)
+        await self.clock.sleep(Duration(seconds=1))
         return HTTPStatus.OK, {"result": True}