Toggle SAFE syncmode on and off automatically

If monerod is started with default sync mode, set it to SAFE after
synchronization completes. Set it back to FAST if synchronization
restarts (e.g. because another peer has a longer blockchain).

If monerod is started with an explicit sync mode, none of this
automation takes effect.

src/cryptonote_core/cryptonote_core.cpp

@@ -316,7 +316,7 @@ namespace cryptonote
 
     const std::string filename = folder.string();
     // default to fast:async:1
-    blockchain_db_sync_mode sync_mode = db_async;
+    blockchain_db_sync_mode sync_mode = db_defaultsync;
     uint64_t blocks_per_sync = 1;
 
     try
@@ -349,11 +349,15 @@ namespace cryptonote
           sync_mode = db_nosync;
         }
         else if(options[0] == "fast")
+        {
           db_flags = DBF_FAST;
+          sync_mode = db_async;
+        }
         else if(options[0] == "fastest")
         {
           db_flags = DBF_FASTEST;
           blocks_per_sync = 1000; // default to fastest:async:1000
+          sync_mode = db_async;
         }
         else
           db_flags = DEFAULT_FLAGS;