Implement postage stamps

[holisticode]

Postage stamps is a major feature for Swarm 1.0. It thus has high priority for implementation.

Currently, some high level descriptions and a spec exist for postage stamps:

• The epic: [EPIC] Postage stamps user-stories#61
• First high level user story: [User-Story] Payment and Upload with Postage Stamps user-stories#62
• SWIP (spec): https://github.com/ethersphere/SWIPs/blob/master/SWIPs/swip-8.md
• Contract: https://github.com/ethersphere/swarm-system-contracts/blob/master/contracts/Postage.sol

This issue is about the actual implementation task for the go-side.

There is still some uncertainty about the details of implementing postage stamps. They should be added progressively here and/or in the official documentation for successful implementation.

[IxaBrjnko]

404... https://github.com/ethersphere/swarm-system-contracts/blob/master/contracts/Postage.sol