Basic-auth improvements

* If a TDRemoteRequest gets an auth challenge and finds a new credential based on the Realm string,
  it now remembers that in its authorizer, and the replicator will pick it up and use that as its
  authorizer from then on. This should help pick up credentials registered with non-empty realms.
* TDRemoteRequest now tries the 'proposed' credential, if there is one, on its first pass through
  the auth challenge handler.
* Added more logging about the auth flow, for future debugging needs.

I am not sure if these will fix issue #242 but they should help.
Some correctness tweaks to TDSocketChangeTracker

A futile attempt to address truncated feeds (#241); but I think these are
good changes to make anyway even if they didn't help that.