Enable contractWatcher without prior headerSync

[rmulhol]

• Previous setup would fail if there were no headers in the db. This
  makes sense because we need headers that haven't been checked for
  logs to exist so that we can fetch logs for those headers. But it
  also prevents us from kicking off the headerSync and contractWatcher
  processes concurrently. These changes enable kicking off both
  processes at the same time with the idea that we will have unchecked
  headers upon transformer execution.

Resolves #155

@i-norden let me know if I'm missing unforeseen risks to enabling concurrent execution - ran into the issue while working on a docker deploy that execute both processes

[elizabethengelman]

LGTM.

At first it felt like we would miss something at the beginning before headers are populated... but I think that those headers will be returned in the next MissingHeadersForAll call.

[i-norden]

LGMT and I can't spot any issues with this, MissingHeadersForAll will just return empty slice of headers until some can be found in the db and tr.Start won't update until it has processed headers. Thanks for catching this!