This fixes a critical bug in the storage node that ignored read locks,
so transactions that have just been committed could be missing in the
results.

Because read locks are released by order of tid, a single storage node
never returned a list with holes (i.e. the missing transactions could
only be the last ones). However, holes could happen when merging
results from multiple storage nodes.

The change in the client node avoids extra RPC at the end:
it should speed up in most cases and it makes the new test clearer.