Add support of async execution

[ziflex]

We need to add possibility execute queries asynchronously i.e. not blocking HTTP request.

The design is pretty simple:

• Add [POST] /async (or other name, naming is hard :)) method that receives similar payload to [POST] / with additional property .callback that represents a valid URL (we need to validate it before responding):

type AsyncScript struct {
    Script
    Callback string `json: "callback"`
}

• If everything is ok, the output must be an uuid value representing a job ID.
• Once Worker received an async payload, it schedules it internally using thread-pool.
• When Worker is done with a job, whether it failed or not, it should call a given callback url with a following payload

type AsyncResult struct {
    Status string // succeeded or failed
    Data byte[] // script result or error
}