Enable background refresh for the scan server tablet metadata cache

[cshannon]

This adds a property to configure the scan server tablet metadata Caffeine cache to refresh cached tablet metadata in the background on cache hits after the refresh time has passed. The refresh time is expressed as a percentage of the expiration time. This allows the cached entries to refresh before expiration if they are frequently used so that scans will not be blocked waiting on a refresh on expiration. Entries still expire if no cache hits come after the refresh time and expiration time passes.

See: https://github.com/ben-manes/caffeine/wiki/Refresh

This closes #4544

[cshannon]

I should point out that this is targeting 2.1 even though there's a new property because the Scan server properties are all marked experimental anyways. We could wait for 3.1 but this seems beneficial enough to put into 2.1 and is low risk.

[dlmarion]

Reminder to wire up to the Caches class in elasticity

[keith-turner]

Reminder to wire up to the Caches class in elasticity

This was done in merge commit 23ff6460c0e5479f4738219829ce2de6fd3a0951