Introduce Clock.call_when_running(...) to include logcontext by default (#18944)

Introduce `Clock.call_when_running(...)` to wrap startup code in a
logcontext, ensuring we can identify which server generated the logs.

Background:

>  Ideally, nothing from the Synapse homeserver would be logged against the `sentinel` 
>  logcontext as we want to know which server the logs came from. In practice, this is not 
>  always the case yet especially outside of request handling. 
>   
>  Global things outside of Synapse (e.g. Twisted reactor code) should run in the 
>  `sentinel` logcontext. It's only when it calls into application code that a logcontext 
>  gets activated. This means the reactor should be started in the `sentinel` logcontext, 
>  and any time an awaitable yields control back to the reactor, it should reset the 
>  logcontext to be the `sentinel` logcontext. This is important to avoid leaking the 
>  current logcontext to the reactor (which would then get picked up and associated with 
>  the next thing the reactor does). 
>
> *-- `docs/log_contexts.md`

Also adds a lint to prefer `Clock.call_when_running(...)` over
`reactor.callWhenRunning(...)`

Part of https://github.com/element-hq/synapse/issues/18905

tests/server.py

@@ -103,7 +103,7 @@ from synapse.storage.database import LoggingDatabaseConnection, make_pool
 from synapse.storage.engines import BaseDatabaseEngine, create_engine
 from synapse.storage.prepare_database import prepare_database
 from synapse.types import ISynapseReactor, JsonDict
-from synapse.util import Clock
+from synapse.util.clock import Clock
 
 from tests.utils import (
     LEAVE_DB,