Session log message buffer

In some apps the errors are clearly isolated for single session (one http request). Buffer is the way to collect all the information during request serving and make it available when exception occurs and additional debug information needs to be saved somewhere.

```ruby
before do
  GLogg.start_session
end

after do
  GLogg.close_session
end

get '/foo' do
  begin
     process_request
  rescue
    GLogg.close_session(true, GLogg::L_D4).to_json
  end
end
```