analysis: move encodeCU() methods and friends to Entropy

This function only encodes a fully finished CTU; it is performing no analysis
and it needs none of the features or members of the Analysis class. It is much
more clear as a member of the Entropy class.

Note: the way bEncodeDQP is communicated is a huge mess and needs to be cleaned
up soon. And finishCU() can also likely be hugely simplified
ctu: optimize away per-row bitstream coders, no output changes

They were really only being used to hold the proper initial state for each
row. This avoids an entropy load/save per CU in encodeSlice() and while I was
in the area I removed another pile of resetEntropy() calls.  The initial slice
entropy state is calculated once and then kept in the frame encoder for all
future uses.