Make GTTA api call faster

[GalRogozinski]

Description

Make Get Transasctions To Approve faster by utilizing scores.
We will do a weighted random pick from the tip set according to the score.

Motivation

Super fast tip-select

Requirements

1. Make the current (old) GTTA logic still accessible to compass
2. Return two tips by performing a weighted random selection on the tip set.
3. First returned tip should reference the last milestone, the other may or may not.

Open Questions (optional)

1. For requirement 1, we can make a seperate API call that compass will use?
2. For requirement 3, we can make a the second tip always be something that doesn't approve the last milestone to merge left behinds