Node API for starting PoST initialization

[antonlerner]

Overview / Motivation

The wallet needs to be able to start the creation of PoST proof

The Task

Add RPC for PoST initialsation

Implementation Notes

TODO: Add links to relevant resources, specs, related issues, etc...

Contribution Guidelines

Important: Issue assignment to developers will be by the order of their application and proficiency level according to the tasks complexity. We will not assign tasks to developers who have'nt introduced themselves on our Gitter dev channel

1. Introduce yourself on go-spacemesh dev chat channel - ask our team any question you may have about this task
2. Fork branch develop to your own repo and work in your repo
3. You must document all methods, enums and types with godoc comments
4. You must write go unit tests for all types and methods when submitting a component, and integration tests if you submit a feature
5. When ready for code review, submit a PR from your repo back to branch develop
6. Attach relevant issue to PR

[zalmen]

The API should support both slow and fast init modes