Add milestone stage

[GalRogozinski]

Description

Make milestone tracker and milestone solidifier and latest milestone tracker a single stage in the TXPipelining Process

Motivation

The first part of #1674

Requirements

1. Make Transaction Tracker load milestones from DB only upon startup
2. Transaction tracker will pass valid milestones to the milestone solidifier queue that will utilize the Transaction Solidifier (no longer a stage)
3. The next stage afterwards will be the broadcasting stage
4. We solidify milestones in order according to index

[GalRogozinski]

Implementation detail:
The milestone solidifier service will have a bootstrapping thread that will die after it loaded the needed milestones