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/shard_map.cc

@@ -209,4 +209,34 @@ void ShardManager::update_shard(Shard& shard, std::string user)
     {
         m_maps[user] = shard;
     }
+
+    mxb_assert(m_limits[user] > 0);
+    --m_limits[user];
+}
+
+void ShardManager::set_update_limit(int64_t limit)
+{
+    std::lock_guard<std::mutex> guard(m_lock);
+    m_update_limit = limit;
+}
+
+bool ShardManager::start_update(const std::string& user)
+{
+    bool rval = false;
+    std::lock_guard<std::mutex> guard(m_lock);
+
+    if (m_limits[user] < m_update_limit)
+    {
+        ++m_limits[user];
+        rval = true;
+    }
+
+    return rval;
+}
+
+void ShardManager::cancel_update(const std::string& user)
+{
+    std::lock_guard<std::mutex> guard(m_lock);
+    mxb_assert(m_limits[user] > 0);
+    --m_limits[user];
 }