add two methods to SpdyTimer:

 - isValid
 - fireDate
in SpdyPersistentRequest, don't schedule a retry if we already have one scheduled.
changes for SpdyPersistentRequest:

 - expose a bunch of retry parameters as properties
 - default these with the preprocessor, not hardcoded
 - add a max retry interval parameter
changes for SpdyPersistentRequest:

 - get rid of reconnectWithMax (always reconnect)
 - make the keepalive handler smarter, don't send a ping if we are not connected