add some logging to SPDY.m
add doConnectCallback to SpdyRequest(Private)
use the new doRequestCallback method in SpdyRequest(Private)
in SpdyRequestCallback
keep some extra connection state in SpdyRequest

prior, there was a window possible where the underlying spdy session had not yet
connected, but we try to do a ping or somesuch.

In this case, we had no state that let us know that we already had tried to reconnect.
some changes for SpdyPersistentUrl:

 - make it a runtime error to set certain inherited callback properties (used internally by this class)
 - be stricter about usage of 'super' vs 'self'
 - use the new isConnecting property of SpdyRequest in sendPing and in reconnect