Revert "adapt fix for race condition in arbiter"

This reverts commit 6876e7a.

entwine/third/arbiter/arbiter.cpp

@@ -353,13 +353,16 @@ std::shared_ptr<Driver> Arbiter::getDriver(const std::string path) const
 {
     const auto type(getProtocol(path));
 
-    std::lock_guard<std::mutex> lock(m_mutex);
-    auto it = m_drivers.find(type);
-    if (it != m_drivers.end()) return it->second;
+    {
+        std::lock_guard<std::mutex> lock(m_mutex);
+        auto it = m_drivers.find(type);
+        if (it != m_drivers.end()) return it->second;
+    }
 
     const json config = getConfig(m_config);
     if (auto driver = Driver::create(*m_pool, type, config.dump()))
     {
+        std::lock_guard<std::mutex> lock(m_mutex);
         m_drivers[type] = driver;
         return driver;
     }