Decided snapshot, add example and remove runtime

[haraldng]

• You ran rustfmt on the code base before submitting (on latest nightly with rustfmt support)
• You reference which issue is being closed in the PR text (if applicable)
• You updated the OmniPaxos book (if applicable)

Issues

Fix #41 , Fix #54, Fix #55

• Snapshots are only applied on decided entries, thus avoiding the trouble having to deal with intermediate snapshots.
• Removed the runtime and used most of the code with tokio as an example for building a KV-store instead.

Breaking Changes

• The structs SequencePaxos and BallotLeaderElection and their interaction are now hidden from the users. Instead, users interact with the OmniPaxos struct that uses one common message queue.
• Replaced the unnecessary tick() with a election_timeout(). Users only need to set a timer and call it, rather than ticking.
• Removed unnecessary constructors for the different message types.

Other Changes

• Added basic snapshot test and updated existing tests to use more proposals.
• Refactored the code of Sequence Paxos into different files for the leader and follower.