Support partial sync on subset of master shards

[adiholden]

Today if not all master shards response with partial sync than we fallback to fullsync. We can improve the logic to enable partial on shards that response with partial and full sync on shards that response with full. To support this we need to make sure we flush the data only for the shards that respond with full sync. What is more complicated is master and replica might not have the same amount of shards so flushing in this case wont be replica shard - this can be implemented with similar logic to flush slot data. Also the sharding logic might change if the dfly version is not the same, in this case we can not know how to flush the shard data.