Fix loopfilter race condition in multithreaded encoder

Race was introduced by https://gerrit.chromium.org/gerrit/15563.

If loopfilter related config params were changed between frames, or
after a KEY frame, there could be a mismatch between encoder's and
decoder's recontructed image. In worst case, when frame buffers are
reallocated because of a size change, the loopfilter could
do an invalid data access (segmentation fault).

Fixes:
Sync with the loopfilter before applying any encoder changes in
vp8_change_config().
Moved the loopfilter synching to the top of
encode_frame_to_data_rate() so that it's done before any alteration of
the encoder.

Change-Id: Ide5245d2a2aeed78752de750c0110bc4b46f5b7b