Refactor GetRelationNodes with following changes
1. Instead of using lappend(NULL, ...) or list_concat(NULL, ...) use
list_make1_int() and list_copy resp.
2. For replicated nodes, return all the nodes for read access. While finalising
the RemoteQuery plans, one of the nodes (preferably preferred one) is picked
from the list of nodes.
3. Remote LOCATOR_TYPE_SINGLE; there is no clue why this locator was added and
it's not used anywhere.
Add a comment about the usage of GTM_CountOpenTransactions

This macro should be used with READ lock hold on gt_TransArrayLock when
called and it is let to the user to do the lock call himself.

Per patch from Nikhil Deshpande