Experimental "PRAGMA query_only=BOOLEAN" statement that is able to turn
write capabilities on and off.
Add the "defer_foreign_keys" pragma and the SQLITE_DBSTATUS_DEFERRED_FKS
value for sqlite3_db_status().  This is a cherry-pick of a sequence of five
checkins in the sessions branch between [1d44e5d3c2] and [d39e65fe70].
Add the experimental "query_only" pragma.
Change the description of how sqlite3_progress_handler() works so that 
the N parameter is "approximate".  This aligns with the current implementation.
This is a documentation change only.  No changes to code.