core/mempool: fix AddBlock and tx pooling concurrency issues

Eliminate races between tx checks and adding them to the mempool, ensure the
chain doesn't change while we're working with the new tx. Ensure only one
block addition attempt could be in progress.

pkg/core/blockchainer.go

@@ -40,6 +40,7 @@ type Blockchainer interface {
 	GetUnspentCoinState(util.Uint256) *UnspentCoinState
 	References(t *transaction.Transaction) map[transaction.Input]*transaction.Output
 	mempool.Feer // fee interface
+	PoolTx(*transaction.Transaction) error
 	VerifyTx(*transaction.Transaction, *block.Block) error
 	GetMemPool() *mempool.Pool
 }