Optional feature (POC) - store values on the external storage (SSD).

1. Add ExternalAllocator that provides files ranges to write to.
2. Add IoMgr that wraps files and allows sending asynchronous requests.
3. Add POC that writes string values when a new entry is added.
   The original values kept are still kept in memory.

src/server/db_slice.h

@@ -159,6 +159,10 @@ class DbSlice {
    */
   size_t FlushDb(DbIndex db_ind);
 
+  EngineShard* shard_owner() {
+    return owner_;
+  }
+
   ShardId shard_id() const {
     return shard_id_;
   }