Default to DB_TXN_WRITE_NOSYNC for all transactional operations

* This is safer than DB_TXN_NOSYNC, and does not appear to impact
  performance.
* Applying this to the dbenv is necessary to avoid many fdatasync(2)
  calls on db 5.x
* We carefully and thoroughly flush databases upon shutdown and
  other important events already.

src/db.h

@@ -216,7 +216,7 @@ public:
         if (!pdb)
             return false;
         DbTxn* ptxn = NULL;
-        int ret = dbenv.txn_begin(GetTxn(), &ptxn, DB_TXN_NOSYNC);
+        int ret = dbenv.txn_begin(GetTxn(), &ptxn, DB_TXN_WRITE_NOSYNC);
         if (!ptxn || ret != 0)
             return false;
         vTxn.push_back(ptxn);