Joining fetch

[RichLogan]

Implement joining fetch.

The idea here, (if others agree), if that joining fetches are so tied to subscriptions that the former is an extension of the latter in terms of the public facing API. When you create a SubscribeTrackHandler you can optionally do so with a joining fetch. Those objects will be returned to your subscribe track handler identically to how live objects would be for convenience. A fetch handler is used internally to make facilitate that mapping, although there are no guarantees about ordering or interleaving with the live stream.

./qserver # Server.
./qclient --pub_namespace=ns --pub_name=name --clock # Publisher
./qclient --sub_namespace=ns --sub_name=name # Regular Subscriber
./qclient --sub_namespace=ns --sub_name=name --joining_fetch # 2nd Subscribe using Joining Fetch (allow time for some objects to be published). 

Fixes #362

[RichLogan]

I am sure there is some state cleanup missing here.