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

synmark/suites/logging.py

@@ -37,6 +37,7 @@ from synapse.logging import RemoteHandler
 from synapse.synapse_rust import reset_logging_config
 from synapse.types import ISynapseReactor
 from synapse.util.clock import Clock
+from synapse.util.duration import Duration
 
 
 class LineCounter(LineOnlyReceiver):
@@ -141,7 +142,7 @@ async def main(reactor: ISynapseReactor, loops: int) -> float:
 
         if len(handler._buffer) == handler.maximum_buffer:
             while len(handler._buffer) > handler.maximum_buffer / 2:
-                await clock.sleep(0.01)
+                await clock.sleep(Duration(milliseconds=10))
 
     await logger_factory.on_done