Take previous reward address in node bootstrap

[samuelmanzanera]

Description

This PR fetch the last reward address to include it into the new node transactions.

Fixes #909

Type of change

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

How Has This Been Tested?

You need to start at least 2 or more nodes.
You need to alter the reward address located in the settings page: /settings
You have to stop the node, and wait the out of sync threshold to force a new transaction of the node's chain (ie. 1 min)
You can restart the node, and it should create a new transaction with the changed reward address.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[tenmoves]

Tested with the method described everything seems fine !