[dbfill] flush stdout for progress output

Flush the progress written to stdout for every 1k documents stored

Change-Id: Id09e61ed8497e53ec94cce7145b40ab6b9d0f357
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/223990
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dan Owen <[email protected]>

Author: trondn

programs/dbfill/dbfill.cc

@@ -373,7 +373,8 @@ int main(int argc, char** argv) {
             auto [node, vb] = node_locator->lookup(key);
             node.enque(std::move(key), vb);
             if ((ii % 1000) == 0) {
-                fmt::print("\rQueued: {}", ii);
+                fmt::print(stdout, "\rQueued: {}", ii);
+                fflush(stdout);
             }
         }