Implement cleanup part 1

By which I mean the queue of transactions that a transactions client created. Since there is a reasonable expectation that a given process will create many transactions instances (in different threads), lets make this one thread and queue static. Some care needs to be taken if different threads use different clusters (including same cluster, different auth). So, maybe one queue/thread per unique cluster?

Also the corresponding hooks, whose lack causes some tests to hang, of course. Lets also look at the fit performer side - is anything needed there apart from implementing the hooks?

FWIW - the lack of cleanup makes some tests fail unless the cluster was recently flushed. Probably some tests are surprised by old txns leaving their turds in the ATR.