Up until now FillDetails, which contains a loop to go through all
the properties requested by the client in a request has been called
at least twice (once for the CFileItem and once for the CFooInfoTag)
for every media item matching the client's request. With n items
and m properties this resulted in n * 2 * m loops which can result
in longer response times. By removing the already matched properties
from the list of requested properties we get down to n * m loops.
Especially for "expensive" properties like "thumbnail" and "fanart"
which require additional SQL queries avoiding doing the same work
twice which can have a huge impact.
In addition moving the logic to retrieve any requested properties
directly from the serialization of the media item instead of first
checking and handling any special cases results in additional
speed-up by not having to do the extra string comparisons and map-
lookups required for the special cases.