Check shard connectivity on AddShard

[Denchick]

Problem: you may add now shards that is not reachably by coordinator or router:

regress=> CREATE SHARD sh5 WITH HOSTS oooops:6432;
    add shard    
-----------------
 shard id -> sh5
(1 row)

For example, on the router startup we check can connect to each shard (no proofs)