util-app: Introduce Flag.getWithDefault

Problem

Clients have no good means of getting back a flag's value
as an Option if it is only provided by it's default.

Solution

Introduce `Flag.getWithDefault` which differs slightly
from `Flag.get` in that it will return a `Some` if the
flag has a default value.

RB_ID=541754
util-stats: Introduces BlacklistStatsReceivers to ignore stats

Problem

Having too many stats entails GC pressure.

Solution

Add a BlacklistStatsReceiver to ignore stats.

RB_ID=542888