pageserver: clean up sharding helper names

[jcsp]

There are various places where we have things like is_zero, is_unsharded. They're also all pub because of being in a different crate than the pageserver code.

[ ] Make a pass over these to make sure the naming is consistent, consider a better name for "is_zero" (as it's not obvious what's special about the zeroth shard), and consider a better name for "is_unsharded" (because it's not obvious that ShardCount(0) (legacy) tenants are distinct from ShardCount(1) (single sharded) tenants).

[ ] Make clear exactly what TenantShardId::unsharded means, and where/why it is safe to use it