Using a CountDownLatch is not composable, which leads to current
layout. Switch to using a SettableFuture, which can be accessed
via AbstractDataStore.initialSettleFuture().
This allows us to externalize the settle policy, letting callers
decide what to actually do.
JIRA: CONTROLLER-1882
Change-Id: Iaf9a359cfc2507ae35688fca3673c13713c2b427
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>