refactor: EventLoop locking cleanups + client disconnect exception

[ryanofsky]

This PR was originally written as a code cleanup to follow up on review comments in earlier PRs. Several other commits were added afterward building on the earlier changes, and necessary for the bugfixes in bitcoin/bitcoin#32345, which resolves issues #123, #176 and #182.

Summary of changes:

• The commit "Improve IPC client disconnected exceptions" is the only external change, and lets clients detect when they are trying to call a remote method after a connection has been closed. This change is used by
  bitcoin/bitcoin#32345 commit "ipc: Handle bitcoin-wallet disconnection" to fix #123.
• The commits "Prevent EventLoop async cleanup thread early exit during shutdown" and "Prevent IPC server crash if disconnected during IPC call" fix issues reported in #182.
• New tests are added in other commits to verify these fixes, covering different kinds of disconnections.
• The change to detect disconnects also relies on an earlier "Add ProxyContext EventLoop* member" commit.
• An "Add EventLoopRef RAII class" commit simplifies a recent bugfix in #159 and also relies on the earlier "EventLoop* member" commit.
• The "Remove DestructorCatcher and AsyncCallable" commit follows up on #144 (comment).
• The "Add clang thread safety annotations" commit follows up on #129 (comment).

[ryanofsky]

Because this change removes EventLoop addClient and removeClient methods it will require an update to Bitcoin core if it is merged.

--- a/src/ipc/capnp/protocol.cpp
+++ b/src/ipc/capnp/protocol.cpp
@@ -41,10 +41,7 @@ class CapnpProtocol : public Protocol
 public:
     ~CapnpProtocol() noexcept(true)
     {
-        if (m_loop) {
-            std::unique_lock<std::mutex> lock(m_loop->m_mutex);
-            m_loop->removeClient(lock);
-        }
+        m_loop_ref.reset();
         if (m_loop_thread.joinable()) m_loop_thread.join();
         assert(!m_loop);
     };
@@ -83,10 +80,7 @@ public:
         m_loop_thread = std::thread([&] {
             util::ThreadRename("capnp-loop");
             m_loop.emplace(exe_name, &IpcLogFn, &m_context);
-            {
-                std::unique_lock<std::mutex> lock(m_loop->m_mutex);
-                m_loop->addClient(lock);
-            }
+            m_loop_ref.emplace(*m_loop);
             promise.set_value();
             m_loop->loop();
             m_loop.reset();
@@ -96,6 +90,7 @@ public:
     Context m_context;
     std::thread m_loop_thread;
     std::optional<mp::EventLoop> m_loop;
+    std::optional<mp::EventLoopRef> m_loop_ref;
 };
 } // namespace

[ryanofsky]

Updated ce4814f -> b47ea9f (pr/eventlock.1 -> pr/eventlock.2, compare), making test and comment fixes, splitting commits, and adding connection disconnects tests and exceptions to help resolve #123

[ryanofsky]

Rebased b47ea9f -> f15ef6c (pr/eventlock.2 -> pr/eventlock.3, compare) so this can be merged cleanly in bitcoin PR bitcoin/bitcoin#32345. Also dropped DisconnectError exception type so original ipc::Exception can be used instead.

[ryanofsky]

Rebased f15ef6c -> 2214358 (pr/eventlock.3 -> pr/eventlock.4, compare) to fix conflict with #165

[ryanofsky]

Rebased 2214358 -> 6715a96 (pr/eventlock.4 -> pr/eventlock.5, compare) to fix conflicts with #172

[ryanofsky]

Rewrote the PR description since this is now a dependency of bitcoin/bitcoin#32345 and needed to fix bugs #123 and #176