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.
Merge pull request #5135

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

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