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/vtab.c

@@ -387,6 +387,7 @@ void sqlite3VtabFinishParse(Parse *pParse, Token *pEnd){
     Schema *pSchema = pTab->pSchema;
     const char *zName = pTab->zName;
     int nName = sqlite3Strlen30(zName);
+    assert( sqlite3SchemaMutexHeld(db, 0, pSchema) );
     pOld = sqlite3HashInsert(&pSchema->tblHash, zName, nName, pTab);
     if( pOld ){
       db->mallocFailed = 1;