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].

FossilOrigin-Name: 527121ac3cdc96ac33ad975c227a6685a2f7e999