datablue: upload endpoint

Data is currently sent to _DataBlue_ via the `poll` endpoint, which stores data as it is received, using the current wall-clock time of the service (i.e., not a supplied time). Now that clients are able to collect data in **Offline** mode, there needs to be a method to send historical data and to supply the time the data was captured, rather than relying on the service to provide timestamps.

It is proposed to add an `upload` endpoint, implemented as a POST method.

- In additional to the standard device query params, such as `ma` and `dk`, the URL query will specify which pins are present and the amount of data associated with each pin (in bytes).
- The body of the request will contain the data for each pin in query param order, where each datum is represented as a `SmallScalar`.

The `SmallScalar` is an eight-byte struct defined as follows:

```
type SmallScalar struct {
Value int32
Timestamp uint32
}
```

Note that unlike the `Scalar` struct, which is used by the datastore, the `SmallScalar` does not require pin/ID info, since the latter info is supplied via the query param.

For example:

`data.cloudblue.org/upload?A4=65536&X60=65536&X61=65536`

would specify that the request contained 65,536 bytes of A4 (battery voltage), X60 (SST), and X61 (BWT) data respectively.

The total POST payload would therefore be 196,608 bytes (= 65,536 x 3). Furthermore, 65,536 bytes of `SmallScalar` data corresponds to 8,192 data points (65,536 / 8). This represents slightly less than six days worth of data, assuming one sample per minute or 1,440 samples per day.