Locking ephemeris structs might hold up decoder

[ljbade]

As discussed with @fnoble and @jacobmcnamee currently we lock on the ephemeris thread the struct each time we calculate the position.

In the lock the struct is copied from the decoder thread, as well as using the copy to calculate the sat position.

Instead we should only copy in the lock, and use the local copy in the calculation outside of the lock.

Even better would be to use the old ephemeris until a new one is ready to not block the decoder at all.