This modifies the ChainTips function in blockchain to return the results
using a type defined in the blockchain package itself instead of
directly returning a dcrjson type. This is preferable since nothing
else in the module depends on dcrjson and therefore allows the
dependency on the dcrjson module to be broken.
It also updates the RPC server that makes use of the function to perform
the necessary conversions.
Finally, the associated module hierarchy documentation is updated to
properly remove the no longer required dependency.
It should be noted that this is a breaking change to the API and thus
will need a v2 major version bump of the blockchain module to be
published before the changes can be externally consumed.
