Checks whether a message identified by the store, folder_id and uid is included in the index
this.
Note the store is not referenced, it's compared pointer-wise, thus when a different instance of the same service as the one
used in the add is used here, the message reference will not be found.
| this | |
| store |
a Store |
| folder_id |
a folder ID, other than zero |
| uid |
a message UID |
|
true when the message reference is included in the index this |