Implement commits full sync

[frolosofsky]

Adds implementation of finalizer-commits based full sync as described in UIP-21.

Also, moves src/finalization/p2p to src/p2p and makes a component from it. Here:

1. Move p2p commits handlers from src/finalizarion/p2p* to src/p2p/finalizer_commits*.
2. Rename commits to FinalizerCommits in type and file names.
3. Pack commits handlers in the component FinalizerCommitsHandler.
4. Adds PushMessage helper function to net_processing.

Fixes #219.

[kostyantyn]

ConceptACK a761adf

There are few comments that should be resolved or answered. Especially the one about replacing CheckFinalizationTx with ContextualCheckFinalizationTx.