add additional entrypoint to TThreadPoolServer with easy exception handling

add additional entrypoint to TThreadPoolServer with easy exception handling

An additional method #serve_rescuable is added, which relays any exceptions
that happen in worker threads to the caller, where they can be handled
directly. 'retry' works too, allowing code like:

  begin
    server.rescuable_serve
  rescue StandardError => e
    puts "got exception: #{e.inspect}"
    ## do something
    retry
  end

The semantics of TThreadPoolServer#serve remain the same.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@668891 13f79535-47bb-0310-9956-ffa450edef68