mempool: simplify names of exported types

With the move to a separate package, naming can be simplified:
MemPool -> Pool, PoolItem -> Item, PoolItems -> Items.

pkg/core/blockchainer.go

@@ -41,5 +41,5 @@ type Blockchainer interface {
 	References(t *transaction.Transaction) map[transaction.Input]*transaction.Output
 	mempool.Feer // fee interface
 	VerifyTx(*transaction.Transaction, *block.Block) error
-	GetMemPool() mempool.MemPool
+	GetMemPool() mempool.Pool
 }