Non-persistent mode for server

[najlkin]

An optional small addition to #336 introducing non-persistent mode of server. It means the server is terminated after all visualization windows are closed. This is useful when you have a single application keeping streams open until it ends and a headless server, which should stop after the data are processed.
For example, when the pause is commented out in ex9.cpp and the visualization output is changed to:

sout << "solution\n" << mesh << u << "\n"
     << "screenshot " << "ex9_" << ti << ".png"
     << endl;

The example can be run like this:

./glvis -hl -no-pr &> log.txt &
./ex9 <parameters...>

producing a screenshot for every visualization step and closing the server afterwards.