preliminary changes/extensions to support replicas

a shardcache node now holds multiple addresses, one for each replica.
The api has been extended to represent such nodes and to simplify initialization

more logic is needed to ensure propagating commands to replicas when necessary
updated the perl bindings to use the new (replica-aware) interface

note that replicas (aka multiple addresses per node) are still not supported in the perl library
ensure releasing the nodes list
documentation
bugfix : don't over-release the list of nodes
first attempt to define an interface for replica support (WIP)
make it build (at least)
some more work on replica support