Refactor sync package to use the strategy approach

[EclesioMeloJunior]

Issue summary

• Currently, Gossamer sync package is full writing for full sync method, so everything in the sync package is a lot coupled and there is no room to expand or introduce a new sync strategy, such as Warp Sync, without a huge code change.

• What I am proposing here is to change the sync package a single SyncService and this service contains a strategy that can be FullSyncStrategy or WarpSyncStrategy both implementing the interface Strategy and all that the SyncService knows is that it has a property strategy of type Strategy.

• The SyncService works in a loop that execs the following actions:

1. Acquire the strategy.NextActions()
2. Execute the next actions
3. Pass the results to strategy.IsFinished()
4. Check if the strategy is done, if so move to the default strategy, otherwise go to step 1

• The implementation of the Strategy interface does not need to have any knowledge about network or workers, it should only know about its current state like: forks, fragments, blocks announced

The approach is explained in this PDF
WARP Sync slides (1).pdf