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/blockchain_db/lmdb/db_lmdb.cpp

@@ -1318,6 +1318,11 @@ void BlockchainLMDB::sync()
   }
 }
 
+void BlockchainLMDB::safesyncmode(const bool onoff)
+{
+  mdb_env_set_flags(m_env, MDB_NOSYNC|MDB_MAPASYNC, !onoff);
+}
+
 void BlockchainLMDB::reset()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);