[Plots] Reset time span with corner reset

On click of corner reset button, reset time
span based on what is observed in settings.
Previously, the actual current time span
was being read, which preserved zoom when
corner reset button was clicked, which is
incorrect. This causes nasa/mct#271.
Merge pull request #276 from VWoeltjen/mct271

[Plots] Reset time span with corner reset
[Persistence] Increase persistence warning time

Wait longer (twenty seconds instead of ten)
before displaying warning message that
persistence is taking too long to initialize;
it is observed, in nasa/mct#274, that this
pop-up may occur during database initialization
on first run on some systems.
Merge pull request #277 from VWoeltjen/mct274

[Persistence] Increase persistence warning time