Fix race conditions in _epggrab_ota_finished() and epggrab_mux_next()

Existing code has race conditions in _epggrab_ota_finished()  and epggrab_mux_next() when two or more adapters are scanning at the same time. See the inline comments in dc75437202d71b79af3b4c85c4d2f6983000d212 for details.