Parse bridge descriptors in a separate thread.
Parse bridge descriptors in a separate thread.

TODO: In the future, we will probably want to add some secondary, temporary
HTTPServer which serves a simple "BridgeDB is down for maintenance and will be
back shortly!" page while the descriptors are being parsed.

 * FIXES #5232: Import bridges into BridgeDB in a separate thread and database
   transaction.

pagea/bridgedb,wfn/bridgedb,wfn/bridgedb,mmaker/bridgedb,mmaker/bridgedb,pagea/bridgedb