Add a description of access rules for the Schema object and lots of asserts

to verify that the access rules are followed.

FossilOrigin-Name: ae8374af057b6e6546e9265148cfffe32fcb0849

src/fkey.c

@@ -1154,6 +1154,7 @@ void sqlite3FkDelete(sqlite3 *db, Table *pTab){
   FKey *pFKey;                    /* Iterator variable */
   FKey *pNext;                    /* Copy of pFKey->pNextFrom */
 
+  assert( db==0 || sqlite3SchemaMutexHeld(db, 0, pTab->pSchema) );
   for(pFKey=pTab->pFKey; pFKey; pFKey=pNext){
 
     /* Remove the FK from the fkeyHash hash table. */