libertem_asi_mpx3: stability and completeness

[sk1p]

Leftover work from #33:

• Active mode - actively set the detector / measurement configuration
  • Complete the serval-client crate, maybe rename to serval-api-client?
  • Keep a persistent HTTP client around in the ServalCient struct, to keep latency low
• Rename crate: it's for all frame-based detectors, not only MPX3
• Robustness: switch to different format, which includes the frame sequence number for handling dropped frames, and possibly better error reporting, acquisition header for easier synchronization
• Some minimal test coverage
• Release on pypi and conda-forge