leveldb: Increase generated keys max length of 'Leveldb external write test' to 50
leveldb: Increase generated keys max length of 'Leveldb external write test' to 50

FactomProject/goleveldb,no2key/goleveldb,btcsuite/goleveldb,ngaut/goleveldb,syndtr/goleveldb,trigrass2/goleveldb,chai2010/goleveldb,zzl0/goleveldb,conseweb/goleveldb,hustbill/goleveldb,rentongzhang/goleveldb