Add X-IC-SetPollDelay or similar

[gabrielbauman]

At times, it might make sense for the server to set the polling rate. This would make it possible for the server to scale down the number of polling requests it has to handle as load increases, and vice versa.

Propose that you include a header like X-IC-SetPollDelay. If the server sets this header to a value (say "30s"), Intercooler would adjust the polling rate for that partial to match.