This commit adds a new API to `Cache` to address data races
with the `TagKeys` and `TagValues` APIs.
`Cache` and `entry` provide `AppendTimestamps`, which
appends the current timestamps to the provided slice
to reduce allocations. As noted in the documentation,
it is the responsibility of the caller to sort and deduplicate
the values, if required.
The `cursors.TimestampArray` type was extended to permit
use of the `sort.Sort` API.
Stuart Carnie