Throttling and/or async queueing machine: how to synchronize IMU and EMG

[iomz]

Related to #51, when selecting the raw EMG mode, the sampling frequencies are different (EMG: 200Hz and IMU: 50 Hz).
The current recorder client generate the timestamp at the reception of the data stream event from dl-myo, but it needs a mechanism to synchronize the two data stream to not push back the aggregated data event stack.
Sometimes it seems jitter reduces the sampling frequency, resulting in the delayed synchronization in a simple FIFO that the current GestureClient implements. It should rather have a more robust synchronization/throttling mechanism to use the device for more than minutes.

[iomz]

After some testing, I propose the following.

1. The base process throughput at 50 Hz, and anticipate pessimistic incoming EMG data with respect to 1 IMU data for 3:1 ratio, meaning it discards every 4th EMG data if any

2. When this 3:1 couldn't even be achieved, drop the data frame

3. The frame protocol stack does the time window thresholding at every frame.

4. Make the above param configurable.