#109 cache HTTPRequest across runs to allow true keep-alive testing

[stephen]

i have 2 distince use cases:

1.each grinder run uses a new HTTP connection and SSL session, here I would use Connection:Close

2.each grinder run uses the same HTTP connection and SSL session across different runs, here I would use Connection:Keep-Alive

currently i have no way of running use case #2 above, because between my test runs, my HTTPConnection is destroyed. i think if the grinder offered me a cache for objects that will persist in memory across runs that would enable me to use it.