Reduce routing info refreshes on the mergeChunks path

 
The current mergeChunks path is very inefficient because:

It performs three sequential refreshes (router, shard-pre-merge and shard-post-merge)

All this makes the mergeChunks command very expensive both from latency and from impact on the config server points of view.

It would be much better if:

• The router command:
  • Didn't do a refresh on entry, but relied on the cached information and the shardVersion (this has backwards compatibility implications)

• The shard command (in order of importance):
  • Just checked the major shardVersion (for routing correctness, i.e., to make sure this shard owns that range)
  • Only did a refresh on the shard if the chunk bounds that the router sent didn't match with the cached info (this can only happen if a previous merge committed against the ConfigServer, but failed to refresh)