[remove-duplicate-repay-recall-topup] adding unique indexes

[thanhnguyennguyen]

fix #301
As Dropdup option is no longer available since mongo 3.X and later (https://stackoverflow.com/questions/30187688/mongo-3-duplicates-on-unique-index-dropdups). We should run following commands in mongodb to remove duplicated documents before restarting fullnode

# remove duplicated repays
db.lending_repays.find({}, {hash:1, txHash:1}).sort({_id:1}).forEach(function(doc){     db.lending_repays.remove({_id:{$gt:doc._id}, hash:doc.hash, txHash: doc.txHash}); });
	
# remove duplicated topups
db.lending_topups.find({}, {hash:1, txHash:1}).sort({_id:1}).forEach(function(doc){     db.lending_topups.remove({_id:{$gt:doc._id}, hash:doc.hash, txHash: doc.txHash}); });

# remove duplicated recalls
db.lending_recalls.find({}, {hash:1, txHash:1}).sort({_id:1}).forEach(function(doc){     db.lending_recalls.remove({_id:{$gt:doc._id}, hash:doc.hash, txHash: doc.txHash}); });