Fix startup race in example

[xyzconstant]

Since #274, connection setup has been split into two calls: MakeStream followed by ConnectStream. Between these calls, the event loop's reference count can temporarily drop to zero, allowing EventLoop::loop() to exit (and the EventLoop to be destroyed) before ConnectStream posts its work. This crashes mpexample on startup.

Fix this by keeping an EventLoopRef alive in main() so the loop stays running while it is in use. This same pattern is already used in Bitcoin Core (m_loop_ref member of CapnpProtocol).

A second commit documents the reference-counted EventLoop lifetime in the EventLoopRef class comment and in doc/usage.md.

NOTE: On macOS, mpexample was crashing intermittently on startup (sometimes failing the m_post_fn == nullptr assert in the EventLoop destructor, sometimes with a mutex lock failure), depending on timing. With this change, the crashes are gone.

[DrahtBot]

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[xyzconstant]

failed CI job seems unrelated

[ViniciusCestarii]

tACK f2fabf6 nice catch! I was able to reproduce on linux and it also intermittently fails without this fix. I liked the doc explanation too.

nit: a more surgical fix for against against master could look like:

+#include "mp/proxy.h"
 #include <init.capnp.h>
 #include <init.capnp.proxy.h>
 
int main(int argc, char** argv)
         loop.loop();
     });
     mp::EventLoop* loop = promise.get_future().get();
+    mp::EventLoopRef loop_ref{*loop};
 
     auto [printer_init, printer_pid] = Spawn(*loop, argv[0], "mpprinter");
     auto [calc_init, calc_pid] = Spawn(*loop, argv[0], "mpcalculator");
int main(int argc, char** argv)
     mp::WaitProcess(calc_pid);
     printer_init.reset();
     mp::WaitProcess(printer_pid);
+    loop_ref.reset();
     loop_thread.join();
     std::cout << "Bye!" << std::endl;
     return 0;