Add concurrent downloads to downloader. (#363)

* Add concurrent downloads to downloader.

* Moved to Go 1.20

* close files channel on producer side.

* Improve error handling

* New flag to ignore signature check results. Improve docs. Do not use number of CPUs to determine number of download workers.

* Set number of default workers in downloader to two.

docs/csaf_downloader.md

@@ -9,9 +9,11 @@ csaf_downloader [OPTIONS] domain...
 Application Options:
   -d, --directory=DIR          DIRectory to store the downloaded files in
       --insecure               Do not check TLS certificates from provider
+      --ignoresigcheck         Ignore signature check results, just warn on mismatch
       --version                Display version of the binary
   -v, --verbose                Verbose output
   -r, --rate=                  The average upper limit of https operations per second (defaults to unlimited)
+  -w, --worker=NUM             NUMber of concurrent downloads (default: 2)
   -H, --header=                One or more extra HTTP header fields
       --validator=URL          URL to validate documents remotely
       --validatorcache=FILE    FILE to cache remote validations
@@ -24,3 +26,8 @@ Help Options:
 Will download all CSAF documents for the given _domains_, by trying each as a CSAF provider.
 
 If a _domain_ starts with `https://` it is instead considered a direct URL to the `provider-metadata.json` and downloading procedes from there.
+
+Increasing the number of workers opens more connections to the web servers
+to download more advisories at once. This may improve the overall speed of the download.
+However, since this also increases the load on the servers, their administrators could
+have taken countermeasures to limit this.