Move database connections map out to separate package (#21207)

The upcoming event main plugin will use a very similar pattern
as the database plugin map, so it makes sense to refactor this and move
this map out. It also cleans up the database plugin backend so that
it does not have to keep track of the lock.

Co-authored-by: Tom Proctor <tomhjp@users.noreply.github.com>

builtin/logical/database/path_config_connection.go

@@ -462,7 +462,7 @@ func (b *databaseBackend) connectionWriteHandler() framework.OperationFunc {
 		b.Logger().Debug("created database object", "name", name, "plugin_name", config.PluginName)
 
 		// Close and remove the old connection
-		oldConn := b.connPut(name, &dbPluginInstance{
+		oldConn := b.connections.Put(name, &dbPluginInstance{
 			database: dbw,
 			name:     name,
 			id:       id,