Don't use heartbeat/wait interval as a flag for a hanging sync.

We can't assume clients using EAS version 12.1 or above actually use
a hanging sync. Introduce a new flag to indicate a hanging sync since
we can't rely on heartbeat/wait interval (these will also be present
when PINGing).

Fixes regression caused by 2a6a67b3aa06af134988d32063c2722eac67508d.
BFN