Replace construction of PathVertex objects with updating existing structure in Crpr computation

This PR:

Modifies clkPathPrev so that instead of calling PathVertex constructor to create temporary object it updates the existing object passed by reference
Updates vertex levels in findCrpr only when necessary.

The results of this modification for Ariane design look as follows (for cts and grt steps, 10 runs):

master cts	PR cts	master grt	PR grt
10:23	11:11	9:45	10:25
11:54	10:57	10:59	10:32
11:44	10:49	11:01	10:52
11:17	11:05	11:18	10:24
11:45	12:36	10:48	12:54
11:35	10:08	10:55	9:37
11:38	10:03	10:44	9:37
11:31	10:03	10:09	9:35
10:25	10:02	9:48	9:39
10:25	10:02	9:50	9:38

This PR could be potentially combined with https://github.com/The-OpenROAD-Project/OpenSTA/pull/211, both affect neighboring parts of code.

The-OpenROAD-Project / OpenSTA