core: re-insert tx into pool, if not confirmed
core: re-insert tx into pool, if not confirmed

In the future, we might want to clean the pool by starting fresh with an
empty pool. Also, if we use a mempool, at any time the generator leader
may exit and forget about our pool tx.

Closes chain/chainprv#2004

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain