Race condition in HandleWork

[bobheadxi]

Run go test -race ./...

--- FAIL: TestHandleWork (0.11s)
        testing.go:730: race detected during execution of test
time="2018-05-11T22:37:50-07:00" level=info msg="Starting Cumulus node"
time="2018-05-11T22:37:50-07:00" level=info msg="Saved new user info to file user.json"
time="2018-05-11T22:37:50-07:00" level=info msg="Created new blockchain with genesis block"
time="2018-05-11T22:37:50-07:00" level=info msg="Starting listener on 127.0.0.1:8080"
time="2018-05-11T22:37:50-07:00" level=debug msg="Worker waiting for work"
time="2018-05-11T22:37:50-07:00" level=info msg="Starting miner"
time="2018-05-11T22:37:50-07:00" level=debug msg="Miner started"
☁️  Welcome to the ☀️  Cumulus console ☁️
EOF
FAIL
coverage: 44.4% of statements