With JOIN from system.clusters one can figure out from which server data
came.
TODO:
- optimization to avoid communicating with unrelated shards (for queries
like "AND _shard_num = n")
- fix aliases (see tests with serverError expected)
v0: AddingConstColumnBlockInputStream
v2: VirtualColumnUtils::rewriteEntityInAst
v3: fix remote(Distributed) by appending _shard_num only if has been requested
