In this commit, we create a new module for `walletdb` in order to allow
external projects to use the project without also having to depend on
the entirety of `btcwallet`. With this commit, projects that depend on
`walletdb` will benefit from a slimmer set of additional dependencies.
Since it's now possible for gozmq.Conn to block when calling Receive,
BitcoindConn hangs upon being stopped because its goroutines are waiting
for a message to be delivered. To address this, we modify it to close
its ZMQ connections driving the goroutines once it's been stopped. This
allows the goroutines to unblock by detecting the EOF error and exiting.
This unifies the logic of receiving an error when broadcasting a
confirmed transaction through btcd's/bitcoind's RPC interface. The btcd
dependency update is required in order for it to match bitcoind's
behavior. For older nodes that have yet to update, the confirmed
transaction will still be caught by the "transaction already exists"
case. This is not needed for bitcoind however, because its been sending
the same RPC error code for several major releases now.
