Update calls to rt:systest_read to handle identical siblings

Update the calls to `rt:systest_read` in `repl_util` and
`repl_aae_fullsync_util` to treat identical siblings resulting from the
use of DVV as a single value.  These changes are specifically to
address failures seen in the `repl_aae_fullsync_custom_n` and
`replication_object_reformat` tests, but should be generally useful for
replication tests using the utility modules that and that have
`allow_mult` set to true.

Also add handling of errors in the AAE index build helper function 
in the `rt` module. AAE fullsync tests can encounter `{error, wrong_node}`
messages resulting from rpc calls to riak_kv_vnode:hashtree_pid. Add 
handling of error responses to `rt:index_built_fun` so that the AAE fullsync 
tests can tolerate this error and continue. This change is necessary to be able 
to verify the proper function of the changes to `rt:systest_read` calls with the two
tests mentioned above.

This change relies on the changes from #646.