Split time too old error into two distinct errors.

This commit splits the two rule validation errors related to the timestamp
being too old into two distince errors rather than grouping them under the
same one.  Thus there is now a new ErrCheckpointTimeTooNew in addition to
the existing ErrTimeTooNew error.

This allows the caller to detect the when a block is rejected due to a
time-related checkpoint failure whereas before the combined error did not.
IMPORT PATH MODIFICATION CHANGESET
----------------------------------

Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.