feat: Repeating operations for util::async (#1776)

Async framework needed a way to do repeating operations (think simplest
cases like AmendmentBlockHandler).
This PR implements the **absolute minimum**, barebones repeating
operations that
- Can't return any values (void)
- Do not take any arguments in the user-provided function
- Can't be scheduled (i.e. a delay before starting repeating)
- Can't be stopped from inside the user block of code (i.e. does not
have stop token or anything like that)
- Can be stopped through the RepeatedOperation's `abort()` function (but
not from the user-provided repeating function)

src/util/Repeat.cpp

@@ -19,24 +19,16 @@
 
 #include "util/Repeat.hpp"
 
-#include <boost/asio/io_context.hpp>
-
 namespace util {
 
-Repeat::Repeat(boost::asio::io_context& ioc) : timer_(ioc)
-{
-}
-
-Repeat::~Repeat()
-{
-    *stopped_ = true;
-}
-
 void
 Repeat::stop()
 {
-    *stopped_ = true;
-    timer_.cancel();
+    if (control_->stopping)
+        return;
+    control_->stopping = true;
+    control_->timer.cancel();
+    control_->semaphore.acquire();
 }
 
 }  // namespace util