Race condition between delete / put for unawaited mutations

[vlovich]

Potentially related to #250, unawaited mutations (put/delete) can race each other even though it can't in the real runtime.

export class Durable {
    constructor(state) {
        this.state = state
    }

    async fetch(request) {
        await this.state.storage.transaction((txn) => {
            void txn.delete('foo')
            void txn.put('foo', 'value')
        })
        return new Response(await this.state.storage.get('foo') ?? 'undefined')
    }
}

export default {
  async fetch(request, env, runtime) {
    const stub = env.DURABLE.get(env.DURABLE.idFromName('some object'))
    return stub.fetch(request)
  }
}