core, dex: polish sync (#75)

- Broadcasting blocks at chain head event is not correct when the full
  node is not running in block proposer mode. Introduce NewFinalizedBlockEvent,
  this event is post by the full node which runs in block proposer mode when a
  block is witnessed and resulting in some blocks are considered finalized.

- Non block proposer node will still broadcast blocks
  at the following moment (same as ethereum):
  1. a sync with a peer is terminated successfully
  2. a block passes the fetcher's header check during inserting blocks
  3. a block is successfully inserted by fetcher

- Don't trigger a sync when we are not behind other peers more than
  acceptable distance. Fetcher is able to cover this.

1 files changed, 3 insertions, 0 deletions

diff --git a/core/events.go b/core/events.go
index 1231daa37..e174e8aad 100644
--- a/core/events.go
+++ b/core/events.go
@@ -32,6 +32,9 @@ type PendingLogsEvent struct {
 // NewMinedBlockEvent is posted when a block has been imported.
 type NewMinedBlockEvent struct{ Block *types.Block }
 
+// NewFinalizedBlockEvent is posted when a block has been imported.
+type NewFinalizedBlockEvent struct{ Block *types.Block }
+
 // RemovedLogsEvent is posted when a reorg happens
 type RemovedLogsEvent struct{ Logs []*types.Log }