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This MR allows a node to export and import chains snapshots.

• Context export/import is added through the lib_storage/context_dump interface.
• The snapshot feature is implemented in lib_shell/snapshots.
• Finally, new node command is added.

This MR depends on !1067 (merged) and must be merge after.

More info can be found in this post: https://blog.nomadic-labs.com/introducing-snapshots-and-history-modes-for-the-tezos-node.html