Make CBlockIndex* returned by GetDepthInMainChain const.

Make the CBlockIndex* (optionally) returned by GetDepthInMainChain
const.  This prevents accidental modification.  The result is for
reading its properties rather than modifying it.
Start with tidier cache directories

Remove more files from the cached, 200-block-chain data directories.
Merge pull request #5139

9f87325 Start with tidier cache directories (Gavin Andresen)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>
Merge pull request #5135

a31e8ba Make CBlockIndex* returned by GetDepthInMainChain const. (Daniel Kraft)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>