Add support for custom provider implementation

- Add a set_provider_factory() method to server_state to allow
  injecting provider factory which will be called when the
  provider is loaded.

Other related changes:
- Implement to_string() helper method for id class.
- Fix id ostream operator human readable id printing.
- Pass victim client_service as an argument to provider::bf_abort()
  to allow passing victim context to custom provider.
- Implement prev() helper method for seqno class.
- Make server_state recover_streaming_appliers_if_not_recovered()
  public. In some recovery scenarios the method must be called outside
  of server_state internal code paths.
- Add storage_service requires_globals() method. The storage service
  implementation may override this to return false if changing to
  storage service context does not require store/reset globals.
- Change view final() to also require that the view status is not
  primary for the view to be final. Also change the method name to
  is_final() to avoid confusion with C++ final identifier.
- Fixes to server state handling in disconnecting and disconnected
  states.
- Keep TOI meta data over whole TOI critical section.

Co-authored-by: Denis Protivensky <denis.protivensky@galeracluster.com>

test/mock_server_state.hpp

@@ -258,12 +258,31 @@ namespace wsrep
                                   rollback_mode)
             , mutex_()
             , cond_()
-            , provider_(*this)
-        { }
+            , provider_()
+        {
+            set_provider_factory([&](wsrep::server_state&,
+                                     const std::string&,
+                                     const std::string&,
+                                     const wsrep::provider::services&)
+            {
+                // The provider object is destroyed upon server state
+                // destruction, so using a raw pointer is safe.
+                provider_ = new wsrep::mock_provider(*this);
+                return std::unique_ptr<wsrep::provider>(provider_);
+            });
 
-        wsrep::mock_provider& provider() const WSREP_OVERRIDE
-        { return provider_; }
+            const int ret WSREP_UNUSED = load_provider("mock", "");
+            assert(ret == 0);
+            assert(provider_ != nullptr);
+        }
 
+        mock_server_state(const mock_server_state&) = delete;
+        mock_server_state& operator=(const mock_server_state&) = delete;
+
+        wsrep::mock_provider& provider() const
+        {
+            return *provider_;
+        }
         // mock connected state for tests without overriding the connect()
         // method.
         int mock_connect(const std::string& own_id,
@@ -308,7 +327,7 @@ namespace wsrep
     private:
         wsrep::default_mutex mutex_;
         wsrep::default_condition_variable cond_;
-        mutable wsrep::mock_provider provider_;
+        wsrep::mock_provider* provider_;
     };
 }