[server][benchmark] Add benchmark for string join case

We often say "for performance" without benchmark. It is not good as a
programmer. We should discuss about performance based on fact. :-)

This commit adds a benchmark that includes benchmark framework
prototype. If we will use fact based discussion about "for performance",
I can extract the benchmark framework into server/mlpl/. We will be able
to create a new benchmark when we have benchmark framework. It helps us
discussing based on fact.

The benchmark is for the following cases:

Precondition:

    std::list<std::string> elements;
    elements.push_back("A");
    elements.push_back("B");
    elements.push_back("C");
    elements.push_back("D");
    elements.push_back("E");

Case1:

    string all;
    all += join(", ", elements);
    all += join(", ", elements);
    all += join(", ", elements);

Case2:

    string all;
    SeparatorInjector injector(", ");
    for (StringListIterator it = m_elements.begin();
         it != m_elements.end();
         ++it) {
            injector(all);
            std::string &element = *it;
            all += element;
    }
    injector.clear();
    for (StringListIterator it = m_elements.begin();
         it != m_elements.end();
         ++it) {
            injector(all);
            std::string &element = *it;
            all += element;
    }
    injector.clear();
    for (StringListIterator it = m_elements.begin();
         it != m_elements.end();
         ++it) {
            injector(all);
            std::string &element = *it;
            all += element;
    }

Here is a result on my environment:

My environment:

  * CPU: Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz
  * configure command line: ./configure --prefix=/tmp/local
    * CXXFLAGS: -g -O2 -O2 -g3

Result:

    % make run-bench-string-join
    ./bench-string-join
                Label:     Total   Average    Median
         string::join: (0.013s)  (0.001ms) (0.001ms)
    SeparatorInjector: (0.007s)  (0.671us) (0.001ms)

The join approach is slower 0.5us than the SeparatorInjector approach.