Allow for concurrent orthanc-anon processing

[stefpiatek]

What happened?

• The api call for orthanc-anon to import from raw doesn't seem to block, so there are multiple requests to CMOVE all concurrently
• There is only ever one CMOVE at a time, so it appears like the CMOVE is a blocking operation
• This then can cause orhtanc-anon to crash

Relevant log output (optional)

No response

What did you expect? (optional)

• Hopefully downloading the zip archive directly from orthanc-raw will not be blocking so that may solve this, if not then I expect the plugin listed in #507 will resolve this
• We should also limit the number of concurrent studies de-identified at once. Republishing the same message to a new queue that orthanc-anon plugin reads could be a good option, then we don't use the API at all but instead read in from that and use the same max in flight?

[p-j-smith]

523 moved the processing in orthanc-anon into background threads using a ThreadPoolExecutor with the number of workers set to PIXL_MAX_MESSAGES_IN_FLIGHT. So hopefully we're not blocking the main thread anymore, and the ThreadPoolExecutor will handle the queuing to ensure only 5 studies are de-identified at any time.

But let's leave the issue open in case we're still having these problems.