Add a timeout on database worker query send.

The number of database worker is limited, and can introduce some
starvation. Print a message each time a two seconds timeout expires while
sending an SQL query to a worker.

* src/cuirass/utils.scm (put-message-with-timeout): New procedure.
(call-with-worker-thread): Rename timeout and timeout-proc arguments to
receive-timeout and receive-timeout-proc arguments. Add two new arguments:
send-timeout and send-timeout-proc. Use put-message-with-timeout to honor
them.
* src/cuirass/database.scm (with-db-worker-thread): Adapt accordingly and pass
send-timeout and send-timeout-proc arguments.