logalloc: relax lifetime rules around region_listener

Currently, a region_listener is added during construction and removed
during destruction. This was done to mimick the old region(region_group&)
constructor, as region_listener replaces region_group.

However, this makes moving the binomial heap handle outside logalloc
difficult. The natural place for the handle is in a derived class
of logalloc::region (e.g. memtable), but members of this derived class
will be destroyed earlier than the logalloc::region here. We could play
trickes with an earlier base class but it's better to just decouple
region lifecycle from listener lifecycle.

Do that be adding listen()/unlisten() methods. Some small awkwardness
remains in that merge() implicitly unlistens (see comment in
region::unlisten).

Unit tests are adjusted.

utils/logalloc.hh

@@ -301,12 +301,14 @@ private:
     const region_impl& get_impl() const;
 public:
     region();
-    explicit region(region_listener& listener);
     ~region();
     region(region&& other);
     region& operator=(region&& other);
     region(const region& other) = delete;
 
+    void listen(region_listener* listener);
+    void unlisten();
+
     occupancy_stats occupancy() const;
 
     allocation_strategy& allocator() noexcept {