lnwallet: add new WithCoinSelectLock method to fix coin select race conditions

In this commit, we add a new method WithCoinSelectLock. This method will
allow us to fix bugs in the project atm that can arise if a channel
funding is attempted (either manually or by autopilot) while a users is
attempting to send an on-chain transaction. If this happens
concurrently, then both contexts will grab the set of UTXOs and attempt
to lock them one by one. However, since they didn't obtain an exclusive
snapshot of the UTXO set of the wallet, they may both attempt to lock
the same input.

We also ensure that calls to SendMany cannot run into this issue by
using the WithCoinSelectLock synchronization when attempting to instruct
the internal wallet to send payments.

rpcserver.go

@@ -804,7 +804,24 @@ func (r *rpcServer) SendMany(ctx context.Context,
 	rpcsLog.Infof("[sendmany] outputs=%v, sat/kw=%v",
 		spew.Sdump(in.AddrToAmount), int64(feePerKw))
 
-	txid, err := r.sendCoinsOnChain(in.AddrToAmount, feePerKw)
+	var txid *chainhash.Hash
+
+	// We'll attempt to send to the target set of outputs, ensuring that we
+	// synchronize with any other ongoing coin selection attempts which
+	// happen to also be concurrently executing.
+	wallet := r.server.cc.wallet
+	err = wallet.WithCoinSelectLock(func() error {
+		sendManyTXID, err := r.sendCoinsOnChain(
+			in.AddrToAmount, feePerKw,
+		)
+		if err != nil {
+			return err
+		}
+
+		txid = sendManyTXID
+
+		return nil
+	})
 	if err != nil {
 		return nil, err
 	}