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/test_server.py

@@ -38,6 +38,7 @@ from synapse.logging.context import make_deferred_yieldable
 from synapse.types import JsonDict
 from synapse.util.cancellation import cancellable
 from synapse.util.clock import Clock
+from synapse.util.duration import Duration
 
 from tests import unittest
 from tests.http.server._base import test_disconnect
@@ -406,11 +407,11 @@ class CancellableDirectServeJsonResource(DirectServeJsonResource):
 
     @cancellable
     async def _async_render_GET(self, request: SynapseRequest) -> tuple[int, JsonDict]:
-        await self.clock.sleep(1.0)
+        await self.clock.sleep(Duration(seconds=1))
         return HTTPStatus.OK, {"result": True}
 
     async def _async_render_POST(self, request: SynapseRequest) -> tuple[int, JsonDict]:
-        await self.clock.sleep(1.0)
+        await self.clock.sleep(Duration(seconds=1))
         return HTTPStatus.OK, {"result": True}
 
 
@@ -423,11 +424,11 @@ class CancellableDirectServeHtmlResource(DirectServeHtmlResource):
 
     @cancellable
     async def _async_render_GET(self, request: SynapseRequest) -> tuple[int, bytes]:
-        await self.clock.sleep(1.0)
+        await self.clock.sleep(Duration(seconds=1))
         return HTTPStatus.OK, b"ok"
 
     async def _async_render_POST(self, request: SynapseRequest) -> tuple[int, bytes]:
-        await self.clock.sleep(1.0)
+        await self.clock.sleep(Duration(seconds=1))
         return HTTPStatus.OK, b"ok"