Add snapshot plugin

[elmato]

This PR adds a new plugin that allows to take a snapshot of the blockchain state at a determined block.
The snapshot is taken after the block is applied and its saved when that block becomes irreversible.

The generated snapshot contains:

• The chain id
• The genesis state
• All accounts (with permissions)
• All the tables

[bytemaster]

The "state" that is saved is not the state of "irreversibility" but the state of a potentially reversible future block.

This plugin should take the snapshot immediately after the referenced block is applied.

The snapshot should be in binary not json.

It will need to save the block_header_state object as well in order to prevent need to replay.

[abourget]

Just so you know.. I also have a solution to freeze a chain at a given block. It's in eosc.. a sort-of eos swiss-knife.

You can run:

$ eosc bps chain-freeze --help
Freeze the chain by proxying p2p blocks until a block including updateauth actions is passed through, then block/shutdown.

Usage:
  eosc bps chain-freeze [flags]

Flags:
      --listening-address string    return producers info in json (default ":19876")
  -t, --target-p2p-address string   return producers info in json (default "localhost:9876")

It listens and sniffs the p2p protocol, and stops propagation, leaving you with a blockchain frozen at a given state. You can then normally query the contracts and all, without fearing it will continue.

[abourget]

whoops, the link is here: https://github.com/eoscanada/eosc or more precisely: https://github.com/eoscanada/eosc/blob/master/eosc/cmd/bpsChainFreeze.go

[elmato]

Replaced by #4837