Make system CallContext load user on demand

Before every thread would hit the database once at the start to
load the system account and user.  The loaded objects are almost never
used.  This change optimizes the behavior and lazy loads the system
user and account object.
Revert "Revert change to ManagedContextTimerTask to unblock DB upgrade"

This reverts commit 2799ee3d4fb86f6db856985879a41e5dfc63ccc6.