bring database holder back to first round generation

[trevjonez]

fixes #979

[andrewforester]

This PR (I used it as SNAPSHOT from develop branch of @trevjones fork) fixes my issue in production. So, waiting for new release of DBFlow with this PR merged.
@agrosner Thank you in advance.

[agrosner]

I believe I moved it back to last round due to ManyToMany generated classes needing to generate the holder in last round to collect all of the files. Its fixed because I do a check on the holder before writing it so we shouldnt run into issues where a blank holder gets written.