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

eth/helper_test.go

@@ -33,7 +33,7 @@ func newTestProtocolManager(blocks int, generator func(int, *core.BlockGen), new
 		evmux       = new(event.TypeMux)
 		pow         = new(core.FakePow)
 		db, _       = ethdb.NewMemDatabase()
-		genesis     = core.WriteGenesisBlockForTesting(db, testBankAddress, testBankFunds)
+		genesis     = core.WriteGenesisBlockForTesting(db, core.GenesisAccount{testBankAddress, testBankFunds})
 		chainman, _ = core.NewChainManager(db, pow, evmux)
 		blockproc   = core.NewBlockProcessor(db, pow, chainman, evmux)
 	)