Avoid crash and/or long delay on shutdown in METAR loader.

Avoid crash and/or long delay on shutdown in METAR loader.
Catching loader thread on subsystem destruction is too late, since it
depends on other subsystems (which are destroyed earlier).
=> Need to stop & join thread during subsystem shutdown.
Also changed loader sleep logic - to avoid excessive delays (up to
30 seconds) on shutdown.
(Issues mostly happened when running offline with realwx enabled.)