This fixes a race condition in makeThread that can currently trigger segfaults
as reported:

bitcoin/bitcoin#34711
bitcoin/bitcoin#34756

The bug can be reproduced by running the unit test added in the previous commit
or by calling makeThread and immediately disconnecting or destroying the
returned thread. The bug is not new and has existed since makeThread was
implemented, but it was found due to a new functional test in bitcoin core and
with antithesis testing (see details in linked issues).

The fix was originally posted in
bitcoin/bitcoin#34711 (comment)

Add test for disconnect race condition in the mp.Context PassField() overload
that can currently trigger segfaults as reported in
bitcoin/bitcoin#34777.

The test currently crashes and will be fixed in the next commit.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

git-bisect-skip: yes

This fixes a race condition in the mp.Context PassField() overload which is
used to execute async requests, that can currently trigger segfaults as
reported in bitcoin/bitcoin#34777 when it calls
call_context.getParams() after a disconnect.

The bug can be reproduced by running the unit test added in the previous commit
and was also seen in antithesis (see details in linked issue), but should be
unlikely to happen normally because PassField checks for cancellation and
returns early before actually using the getParams() result.

This bug was introduced commit in 0174450
which started to cancel requests on disconnects. Before that commit, requests
would continue to execute after a disconnect and it was ok to call getParams().

This fix was originally posted in
bitcoin/bitcoin#34777 (comment)

Add test disconnect for race condition in the mp.Context PassField() overload
reported in bitcoin/bitcoin#34782.

The test crashes currently with AddressSanitizer, but will be fixed in the next
commit. It's also possible to reproduce the bug without AddressSanitizer by
adding an assert:

```diff
--- a/include/mp/type-context.h
+++ b/include/mp/type-context.h
@@ -101,2 +101,3 @@ auto PassField(Priority<1>, TypeList<>, ServerContext& server_context, const Fn&
                     server_context.cancel_lock = &cancel_lock;
+                    KJ_DEFER(server_context.cancel_lock = nullptr);
                     server.m_context.loop->sync([&] {
@@ -111,2 +112,3 @@ auto PassField(Priority<1>, TypeList<>, ServerContext& server_context, const Fn&
                             MP_LOG(*server.m_context.loop, Log::Info) << "IPC server request #" << req << " canceled while executing.";
+                            assert(server_context.cancel_lock);
                             // Lock cancel_mutex here to block the event loop
```

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

git-bisect-skip: yes

This fixes a race condition in the mp.Context PassField() overload which is
used to execute async requests, that can currently trigger segfaults as
reported in bitcoin/bitcoin#34782 when a cancellation
happens after the request executes but before it returns.

The bug can be reproduced by running the unit test added in the previous commit
and was also seen in antithesis (see details in linked issue), but should be
unlikely to happen normally because the cancellation would have to happen in a
very short window for there to be a problem.

This bug was introduced commit in 0174450
which started to cancel requests on disconnects. Before that commit a
cancellation callback was not present.

This fix was originally posted in
bitcoin/bitcoin#34782 (comment)