[eclipselink-dev] Check-in: bug 289019, bug 289020, bug 289021 and bug

[Tom Ware <tom.ware@xxxxxxxxxx>]

https://bugs.eclipse.org/bugs/show_bug.cgi?id=289019
https://bugs.eclipse.org/bugs/show_bug.cgi?id=289020
https://bugs.eclipse.org/bugs/show_bug.cgi?id=289021
https://bugs.eclipse.org/bugs/show_bug.cgi?id=289023

These are changes to allow the creation of a new database platform for the 
symfoware database.

They do the following:

- Check databasePlatform for support before generating constraints
- reduce the precision of some numeric fields on table generators to 18
- Change default sequence generation on one test to get the default sequence 
table name from the platform

These changes are all straighforward and I reviewed the user-suggested fixes as 
I included them and ran our existing tests.

Further testing will come as SymfowarePlatform is constructed.