Implement pkgdb.which for pkgng

Issue:	#41
Rewrite pkgng/pkgdb, load all data once into @db

Issue:	#41
Always consider the db stale if reading from pkgng

Issue:	#41
Use 'pkg' driver when using pkgng

Issue:	#41
Preload all data into @db when using pkgng

Issue:	#41
No longer show USING PKGNG with pkgng
Merge branch '41-pkgng-pkgdb'

* 41-pkgng-pkgdb:
  No longer show USING PKGNG with pkgng
  Preload all data into @db when using pkgng
  Use 'pkg' driver when using pkgng
  Always consider the db stale if reading from pkgng
  Rewrite pkgng/pkgdb, load all data once into @db
  Implement pkgdb.which for pkgng

Issue:	#41
Add NEWS entry on pkgdb+pkgng