Fix two caching-related comments in the code

This patch updates a couple caching-related comments that were
inaccurate or ambiguous.

See also: https://review.openstack.org/#/c/67978

Change-Id: I4772568ff91a7a72f2633a0b4fe45d4f11a2c5da
Rename shard to pool

Most of the work done in https://review.openstack.org/#/c/96463/
and here are for those left.

Change-Id: I9249747557b0784982f2fe75c1f13fd866a16616
Closes-Bug: #1333152
Merge "Fix two caching-related comments in the code"