Price Feeder Oracle Integration

[garyyu]

This PR is used to integrate a Price Feeder Oracle service which use the external Gotts Oracle server as the price data query.

For this major PR, there're at least the following tasks:

• [x] consensus for feeder lists (Floonet testnet at this moment)
• [x] the price data structure which will be used as the p2p broadcasting for price info
• [x] the dedicated lmdb store for price info
• [x] a diff0 compression algorithm for price info compression before integrating into the block header
• [x] a "mining" service to generate the price info for the Price Feeder
• [x] p2p broadcasting for price info
• [x] price info integration into block header, and the decoding
• [x] related tests code.

[garyyu]

There are still two remaining tasks for this Oracle integration, which will be kept as separated PRs:

TODO:

1. Price feeder rewards
2. Prices compression. (Raw ExchangeRates is used in this PR for prices in block)