ListLeasedOutputs can be very slow

[yyforyongyu]

Initially discovered in this issue: lightningnetwork/lnd#8809. Some initial analysis re the bucket lookups when calling ListLeasedOutputs, these buckets are visited at least once, with some of them doing key lookup, while others doing cursor iterations,

bucket: wtxmgrNamespaceKey
	bucket: bucketUnminedCredits
		key: outpoint
		value: []byte
	bucket: bucketUnspent
		key: outpoint
		value: []byte
	bucket: bucketLockedOutputs
		key: outpoint
		value: lockID, expiry
	bucket: bucketUnmined
		key: txhash
		value: wtxmgr.TxRecord
	bucket: bucketUnminedCredits
		key: outpoint
		value: amount
	bucket: bucketUnminedInputs
		key: outpoint
		value: []byte
	bucket: bucketCredits
		key: [72]byte (txhash+op+index)
		value: amount (9 bytes)
	bucket: bucketTxRecords
		key: txhash+wtxmgr.Block
		value: wtxmgr.TxRecord
	bucket: bucketBlocks
		key: block height
		value: [44]byte
	bucket: bucketDebits
		key: ?
		value: wtxmgr.DebitRecord
	bucket: bucketTxLabels
		key: txid
		value: string

Will investigate more to see how we can improve this query.