Introduce 0.25 sec delay for concurrent poll requests

On Servlet containers, requests are processed in concurrent threads
and it's not possible to guarantee the order of processing. This
change introduces a 0.25 sec delay between requests when testing
simultaneous polling requests to maximize the chance of the first
request to be processed first.
Merge remote-tracking branch 'rstoyanchev/connection-still-open'
#65 - also apply to dev