Solidify unsolid transactions encountered during the walk

[GalRogozinski]

Description

while running the walker due to GTTA calls we should solidify transactions we encounter on a separate thread

Motivation

Due to network/node unreliability some transactions may not become solid on a node but they may appear as solid on other nodes.

If the node is public, this may cause users to not have their transactions confirmed...

Requirements

1. Upon calling GTTA we try to solidify unsolid transactions the walker encounters, on a separate thread
2. It shouldn't affect any other API calls (i.e. CheckConsistency)