Queue new block root hash submissions

[karlfloersch]

Is your feature request related to a problem? Please describe.
It seems we may be running into problems with submitRootHash() being called, not completing, and then being called again. Because we are not providing any nonce management, this is sure to cause issues.

Describe the solution you'd like
A very simple temporary fix is adding a submitRootHashQueue similar to the one found in https://github.com/plasma-group/plasma-chain-operator/blob/master/src/block-manager/block-store.js#L51-L59 . The code would be something like:

  this.rootHashQueue = []

  async submitRootHash (rootHash) {
    log('Submitting root hash:', rootHash)
    const deferred = defer()
    this.rootHashQueue.push({ rootHash, resolve: deferred.resolve })
    if (this.rootHashQueue.length === 1) {
      this._processRootHashQueue()
    }
    return deferred.promise
  }

  async _processRootHashQueue () {
    let numRootHashProcesses
    for (numRootHashesProcessed = 0; numRootHashesProcessed < this.rootHashQueue.length; numRootHashesProcessed++) {
      this.rootHashQueue[numRootHashesProcessed].receipt = await es.plasmaChain.methods.submitBlock('0x' + this.rootHashQueue[numRootHashesProcessed].rootHash).send({gas: 400000, gasPrice: '5000000000'})
    }
    const processedRootHashes = this.rootHashQueue.splice(0, numRootHashesProcessed)
    for (const processedRootHash of processedRootHashes) {
      processedRootHash.resolve(processedRootHash.receipt)
    }
  }

Note that I just copied and pasted the code from block-manager and changed variable names. Probably should extract this logic into some kind of library & share code between them.

Describe alternatives you've considered
Eventually we probably want to store every transaction that is sent out of the operator & the latest nonce. This way we can have a full history of transactions which are sent and can ensure that we never send two txs with the same nonce. And if one tx we signed isn't getting through, we can add logic for rebroadcasting.