Rename ua_session of HttpSM

[masaori335]

After TS-3612, ua_session is ProxyClientTransaction * now. This made confusion very much.
It'd be great if this is renamed to ua_transaction.

trafficserver/proxy/http/HttpSM.h

Line 334 in 4d4283e

ProxyClientTransaction *ua_session = nullptr;

[shinrich]

Yep, seems like a good idea.