[server][benchmark] Remove reserving memory from fast_join

It is unfair.
We should reserve memory for all cases or don't reserve.

Before:

    % make -s -C server/benchmark run-bench-string-join
                Label:     Total   Average    Median
         string::join: (0.011s)  (0.001ms) (0.001ms)
    string::fast_join: (0.005s)  (0.500us) (0.000us)
    SeparatorInjector: (0.005s)  (0.529us) (0.001ms)

After:

    % make -s -C server/benchmark run-bench-string-join
                Label:     Total   Average    Median
         string::join: (0.008s)  (0.816us) (0.001ms)
    string::fast_join: (0.006s)  (0.577us) (0.001ms)
    SeparatorInjector: (0.006s)  (0.576us) (0.001ms)

With this result, we know that we can get the similar performance with
fast_join or SeparatorInjector.

I will send a propose that replacing SeparatorInjector with
fast_join. Because user code of fast_join approach is simpler rather
than SeparatorInjector approach and no performance difference.