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/prepare.c

@@ -470,6 +470,7 @@ static void schemaIsValid(Parse *pParse){
     ** value stored as part of the in-memory schema representation,
     ** set Parse.rc to SQLITE_SCHEMA. */
     sqlite3BtreeGetMeta(pBt, BTREE_SCHEMA_VERSION, (u32 *)&cookie);
+    assert( sqlite3SchemaMutexHeld(db, iDb, 0) );
     if( cookie!=db->aDb[iDb].pSchema->schema_cookie ){
       pParse->rc = SQLITE_SCHEMA;
     }