MXS-3892: Limit concurrent mapping of databases

As there are no practical benefits to multiple sessions for the same user
mapping the databases at the same time, limiting them to one update per
user is sensible. This is especially true now that we know the
information_schema tables aren't the most efficient things in the world.

The current code implements this rate limiting by closing any extra
sessions that would start a second update. The final implementation should
suspend them for the duration of the update as it is far more
user-friendly.

The limits are currently global as the shard caches are also global. This
is a performance bottleneck and it could be solved by storing the shard
cache inside of a mxs::WorkerGlobal instead of having it as a global
cache.

server/modules/routing/schemarouter/schemaroutersession.hh

@@ -160,6 +160,7 @@ private:
     SSRBackendList         m_backends;      /**< Backend references */
     SConfig                m_config;        /**< Session specific configuration */
     SchemaRouter*          m_router;        /**< The router instance */
+    std::string            m_key;           /**< Shard cache key */
     Shard                  m_shard;         /**< Database to server mapping */
     std::string            m_connect_db;    /**< Database the user was trying to connect to */
     std::string            m_current_db;    /**< Current active database */