txm: free read lists of a transaction when it's prepared

Read lists (read set and other similar lists) are used only for
detecting a conflict when another transaction is committed.
Once a transaction is prepared (no matter with success or not)
those lists are no more needed. Moreover, in some part of code it
is expected that there can be no read set of already prepared tx.

So let's clean those lists once a transaction is prepared.

Closes #7945

NO_DOC=bugfix