BUG-8618: make sync threshold tuneable

We are observing quite a few of these transitions, which may be coming
from batching scenarios. Introduce sync-index-threshold config knob
to expose control over it.

Change-Id: Ief4c89c2fe5b95cebaf3fb83cbcdda37cac126b6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 890e4bbf40aee318a2174bd4130cf34437e5617b)