The bug was that we accessed static class members, namely the two maps
NetworkStatusEntryImpl#flagIndexes and #flagStrings, during instance
creation without synchronization. This worked just fine with a single
thread creating instances, but it breaks with multiple threads doing
that at the same time.

The fix is to keep a separate map per NetworkStatusImpl instance and
share that between all its NetworkStatusEntryImpl instances. This
doesn't save as much memory as sharing maps between all
NetworksStatusEntryImpl instances ever created, but it's a reasonable
compromise between memory and runtime efficiency. In contrast to that,
synchronizing map access would have put a major runtime performance
penalty on parsing.

Fixes #32194.

Also extend DescriptorReader#readDescriptors to support .gz-compressed
files which will be necessary to process files rsync'ed from BridgeDB.
And maybe it's useful for other purposes, too.

Implements part of #19332.