database: Warn if the worker is busy for more than 5 seconds.

* src/cuirass/utils.scm (with-operation, get-message-with-timeout): New
procedures,
(call-with-worker-thread): add timeout and timeout-proc arguments.
* src/cuirass/database.scm (with-db-worker-thread): Pass a 5 seconds timeout
to call-with-worker-thread, and print a debug message on timeout expiration.