add 'Close() err' to TransactionPool interface
add 'Close() err' to TransactionPool interface

rivine/rivine,NebulousLabs/Sia,MrStrong/Sia,ChrisSchinnerl/Sia,mharkus/Sia,mnsl/Sia,mtlynch/Sia,ericflo/Sia,mnsl/Sia,mtlynch/Sia,MrStrong/Sia,rivine/rivine,NebulousLabs/Sia,ericflo/Sia,mharkus/Sia,ChrisSchinnerl/Sia