peer: avoid blocked writeHandler<->queueHandler interaction
peer: avoid blocked writeHandler<->queueHandler interaction

This commit addresses an issue that could occur if a
message was attempted added to the sendQueue by the
queueHandler before the writeHandler had started.

If a message was sent to the queueHandler before the
writeHandler was ready to accept messages on the
sendQueue, the message would be added to the
pendingMsg queue, but would not be attempted sent
on the sendQueue again before a new incoming message
triggered a new attempt.

aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd