Implement light client sync algorithm

[StoyanDrumev]

[[Implement grandpa justifications verification algorithm]](https://spec.polkadot.network/#warp-sync) using the info coming from /sync/warp
Implement a sync query process based on this [algorithm](https://spec.polkadot.network/#algo-light-clients-query-state) because it’s used by the warp sync algorithm
Persist latest finalized block information to DB

[vikinatora]

This big task can be split into several smaller ones:

• Find snapshot with light client state, add it, read it
• Implement authority set change verification algorithm
• Implement grandpa justification verification algorithm
• Implement light client response verification algorithm and merkle proof against the Commit Root Hash from the warp sync response. See here for more info.
• Implement the light client sync algorithm based on this and this diagrams.

@Kirqkas These should be separate issues I think.

[Kirqkas]

• Find snapshot with light client state, add it, read it Find snapshot with light client state, add it, read it #80
• Implement authority set change verification Implement authority set change verification algorithm #81
• Implement grandpa justification verification Implement grandpa justification verification algorithm #82
• Implement light client response verification algorithm and merkle proof against the Commit Root Hash from the warp sync response. See here for more info. Implement light client response verification algorithm and merkle proof against the Commit Root Hash from the warp sync response.  #83
• Implement the light client sync algorithm based on this and this diagrams. Implement the light client sync algorithm  #84
• Fix heap pages reading bug #102