USE_DB_PROPS define

[Hyrum K. Wright]

To anybody concerned:
We currently use the USE_DB_PROPS define to filter out the experimental use
of exclusive in-db-properties.  Since that will be implemented in format 17,
which is imminent as soon as Greg is home from holiday, I'd like to change
the defines from

#ifndef USE_DB_PROPS

to

#if (SVN_WC__VERSION < SVN_WC__PROPS_IN_DB)

The rationale is that they really just mean the same thing.  I'll shortly be
testing the pristine upgrade mechanism, and would like to do a similar thing
for the (albeit more experimental) pristine changes slated for format 18.
Using the format number rather than arbitrary defines makes this testing
much easier.

-Hyrum