picopb version and date/number epochs

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

devel/picopb, software I wrote several years ago and haven't properly
released, has a version number of 20150628.

It is unlikely that anything actually uses picopb (yet).  (If you do
use this, I'm curious to hear -- please let me know!)

If I do release a picopb version, I will probably want to have
~semantic API and ABI version numbering scheme to detect skew between
the library and generated file.

Is it OK if I just change this to 0.0.0a20150628?

For other packages, perhaps we should generally have a rule that
date-based prereleases should look like that, so we don't have to
worry about version epochs on the first number-based release, unless
upstream deliberately uses date-based versioning long-term?