pageserver: automated rename of Tenant -> TenantShard

[jcsp]

Problem

This has been a pending cleanup since we added sharding.

Historically, Tenant represented a whole tenant, but these days it only represents one shard of a tenant. This is confusing for any first-time reader of the codebase.

Summary of changes

• Rename Tenant -> TenantShard
• Rename TenantManager -> TenantShardManager
• Rename a couple of helper types (TenantsMap, TenantHarness)

This isn't totally comprehensive, but addresses the most intrusive piece of confusing naming (the Tenant type not representing a whole tenant).

[github-actions]

7326 tests run: 6948 passed, 0 failed, 378 skipped (full report)

---

Flaky tests (2)

Postgres 17

• test_storage_controller_node_deletion[True]: debug-x86-64

Postgres 16

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64

Code coverage* (full report)

• functions: 33.7% (8425 of 25029 functions)
• lines: 49.2% (70500 of 143376 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
652479c at 2025-01-16T16:46:02.875Z :recycle:}