database: Set a 'busy_timeout' to handle concurrent accesses.

Fixes a bug whereby some fibers would get a SQLITE_BUSY exception while
accessing the database: see
<https://debbugs.gnu.org/cgi/bugreport.cgi?bug=30644#26>.

Suggested by Danny Milosavljevic <dannym@scratchpost.org>.

* src/cuirass/database.scm (wal-mode): Rename to...
(set-db-options): ... this.  Add call to 'sqlite-exec' for
'busy_timeout'.