Make ARS use causally consistent ShardRegistry::getShard() function

Currently the ARS uses the non-causally consistent ShardRegistry::getShardNoReload() function to retrieve the target shard.

This function doesn't guarantee that:

• the ShardRegistry has been properly initialized
• the ShardRegistry contains the information related to the last gossiped topology time

The ARS should use the causally consistent counter-part function  ShardRegistry::getShard().

A first attempt to fix this showed that is not feasible to call this blocking function using the internally stored operation context. As alternative solution we could provide a async version of the ShardRegistry::getShard that will be used in the ARS future-chain that implements the remote request scheduling.