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.
In this commit, we fix a small issue where it's possible that we read a
malformed message from the ZMQ connection to bitcoind due to it shutting
down. To fix this, we ensure that the event type is human readable
before attempting to log it.
