message: indexer-service: Look up state channels via a string key
  The problem with using an object is that it doesn't seem
  to hash/compare properly, causing lookups to fail sometimes.
diff: @@ -245,11 +245,6 @@ export interface PaymentManagerCreateOptions {
  metrics: metrics.Metrics
  }
  -interface StateChannelKey {
  - subgraph: string
  - epoch: number
  -}
  -
  export class PaymentManager extends EventEmitter<PaymentManagerEventNames>
  implements PaymentManagerInterface {
  options: PaymentManagerOptions
  @@ -257,7 +252,7 @@ export class PaymentManager extends EventEmitter<PaymentManagerEventNames>
  logger: logging.Logger
  wallet: Wallet
  epoch: number
  - stateChannels: Map<StateChannelKey, StateChannelInterface>
  + stateChannels: Map<string, StateChannelInterface>
  constructor(options: PaymentManagerOptions) {
  super()
  @@ -275,7 +270,7 @@ export class PaymentManager extends EventEmitter<PaymentManagerEventNames>
  let queue = new PQueue({ concurrency: 2 })
  for (let subgraph of subgraphs) {
  - let key = { subgraph, epoch: this.epoch }
  + let key = `${this.epoch}/${subgraph}`
  queue.add(async () => {
  if (!this.stateChannels.has(key)) {
  @@ -301,7 +296,7 @@ export class PaymentManager extends EventEmitter<PaymentManagerEventNames>
  let queue = new PQueue({ concurrency: 2 })
  for (let subgraph of subgraphs) {
  - let key = { subgraph, epoch: this.epoch }
  + let key = `${this.epoch}/${subgraph}`
  queue.add(async () => {
  if (this.stateChannels.has(key)) {
  @@ -316,6 +311,6 @@ export class PaymentManager extends EventEmitter<PaymentManagerEventNames>
  }
  stateChannelForSubgraph(subgraph: string): StateChannelInterface | undefined {
  - return this.stateChannels.get({ subgraph, epoch: this.epoch })
  + return this.stateChannels.get(`${this.epoch}/${subgraph}`)
  }
  }
  