Previously, HealthMvcEndpoint stored the cached Health and its last
access time in two separate fields. Neither field was volatile and
no synchronization was used. This meant that there were potential
visibility problems. In a possible worst case scenario one field may
see the updated access time but an old health so it would incorrectly
believe that the old health was up-to-date and return it.

This commit reworks the endpoint to store the cached health and the
time at which it was created in a single, volatile field. This ensures
that the cached health and its creation time will be visible across
threads. Note that a race between threads when the cache is stale is
still possible. This race may result in multiple calls to the
delegate but these should be harmless.

Closes gh-9454