motor signal rate causing long delay

Created by: meguiraun

https://github.com/mxcube/mxcube3/pull/373 introduced a limit on the number of signals sent per second. However, if there are many signals in a very small period there will be all delayed and the client will still receive signals long after the movement is finished.

Rough test with our diff: 90deg rotation -> ~10 signals/second, and the movement takes ~1 sec

Could we discard the signals arriving inside the delay interval? So if we have 3 sig/sec as limit the client will only receive 3/4 signals, and all will arrive ~synchronized with the movement.