core, core/types: readd transactions after chain re-org

Added a `Difference` method to `types.Transactions` which sets the
receiver to the difference of a to b (NOTE: not a **and** b).

Transaction pool subscribes to RemovedTransactionEvent adding back to
those potential missing from the chain.

When a chain re-org occurs remove any transactions that were removed
from the canonical chain during the re-org as well as the receipts that
were generated in the process.

Closes #1746

core/events.go

@@ -36,6 +36,9 @@ type NewBlockEvent struct{ Block *types.Block }
 // NewMinedBlockEvent is posted when a block has been imported.
 type NewMinedBlockEvent struct{ Block *types.Block }
 
+// RemovedTransactionEvent is posted when a reorg happens
+type RemovedTransactionEvent struct{ Txs types.Transactions }
+
 // ChainSplit is posted when a new head is detected
 type ChainSplitEvent struct {
 	Block *types.Block