[Persistence] Use delay when scheduling component updates

Issue nasa/mct#198 results from attempts to lookup the
timestamp from the database made while the database is
not fully initialized. This occurs in timed task which
updates changed components from the database.
Scheduling this task with a delay helps to avoid this
situation.
[Persistence] Add a minimum polling interval

Add a minimum interval at which the database will
be polled. This avoids the situation in which the
user configures a polling interval short enough
to reproduce nasa/mct#198 (or a polling interval
of 0, which may simply cause MCT to lock up.)
[Persistence] Catch exception when reading database time

In cases where polling begins before database is
fully initialized (which may still occur for very
short polling intervals) catch the resultant
exception and simply return null (method already
uses null to indicate that no result is
available.)
Merge pull request #211 from VWoeltjen/mct198

[Persistence] Avoid polling uninitialized database