frameencoder: re-order compressFrame() to avoid dependency problem with HRD

1. if --hrd is enabled, the buffer period SEI needs data set by rateControlStart
2. rateControlStart may block, so get compute heavy tasks done first, like
   weighted prediction analysis
3. hoist rateControlStart and QP setup back out of compressCTURows() and move
   other code into it that belonged there in the first place
4. move framefilter setup logic together
5. don't measure rateControlEnd() in frame wall time
frameencoder: nits