Fixed stat collection for getTxToApprove

[gjeee]

Description

The gatherStatisticsOnTipSelection() routine only updated the counterGetTxToApprove variable and not the elapsedTime_getTxToApprove variable.

I fixed this and took the opportunity to reorganize and simplify the stat collecting routines (i added a similar stat routine for the proof-of-works). Further I updated a typo (ellapsed -> elapsed).

Fixes #1060

Type of change

• Bug fix (a non-breaking change which fixes an issue)

How Has This Been Tested?

N/A

Checklist:

• [x] My code follows the style guidelines for this project
• [x] I have performed a self-review of my own code
• [x] New and existing unit tests pass locally with my changes