Feature Request: Add FindAllTabletsInShard function that operates like FindAllShardsInKeyspace

[mattlord]

Feature Description

It's a common pattern today to:

1. Get a list of tablet names in a shard
2. Loop over that list and get each tablet record with that name

We could cut the topo calls in that case e.g. from 4 to 1 (1 each of primary, replica,rdonly) by mirroring what we did in FindAllShardsInKeyspace — attempting to get all of the tablet records in a range scan if we can, and if not fall back to getting the tablet records concurrently.

Use Case(s)

Lessening load on topo servers.