Fix shutdown deadlock for ws notification queue.
Fix shutdown deadlock for ws notification queue.

While here, correct the comment on rpcServer.notificationQueue.

Closes #122.

Roasbeef/btcwallet,decred/dcrwallet,jrick/dcrwallet,Stash-Crypto/btcwallet,decred/dcrwallet,synrg-labs/btcwallet,jrick/dcrwallet,synrg-labs/btcwallet,jrick/btcwallet,btcsuite/btcwallet,btcsuite/btcwallet,Stash-Crypto/btcwallet,jrick/btcwallet,Roasbeef/btcwallet