Provide statistics on the number of retry attempts

[jd]

That could be useful to be able to trace things, etc.

[harlowja]

Ya, that'd be nice,

    @abc.abstractproperty
    def statistics(self):
        """A dictionary of runtime statistics this engine has gathered.
        This dictionary will be empty when the engine has never been
        ran. When it is running or has ran previously it should have (but
        may not) have useful and/or informational keys and values when
        running is underway and/or completed.
        .. warning:: The keys in this dictionary **should** be some what
                     stable (not changing), but there existence **may**
                     change between major releases as new statistics are
                     gathered or removed so before accessing keys ensure that
                     they actually exist and handle when they do not.
        """

We can use a similar docstring :-P (the above is from a taskflow engine).

[crowdwave]

statistics has moved, see

#486