Move the AVPacketQueue functionality that is currently only used
for the decklink decode module into decklink_common, so it can
be shared by the decklink encoder (i.e. for VANC insertion when
we receive data packets separate from video).

The threadsafe queue used within the decklink module was named
"AVPacketQueue" which implies that it is part of the public API,
which it is not.

Rename the functions and the name of the queue struct to make
clear it is used exclusively by decklink, per Marton Balint's
suggestion.
Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>