Cross-DB cleanups and Postgres support.

[Robin H. Johnson]

The following patchset contains a number of cleanups for cross-database
support, and then finally a cleaned up version of the Postgres Store as posted
by Radu Greab in January.

It passes both the testsuite with both the MySQL and Postgres Stores.

Diffstat of cleanups:
 server/lib/MogileFS/Store.pm         |   46 +++++++++++++++++++----------------
 server/lib/MogileFS/Worker/Delete.pm |    2 -
 server/lib/MogileFS/Worker/Query.pm  |   10 +++----
 3 files changed, 30 insertions(+), 28 deletions(-)

Diffstat of postgres patch [7/7]
 server/lib/MogileFS/Store/Postgres.pm |  550 ++++++++++++++++++++++++++++++++++
 server/t/00-startup.t                 |    2 
 server/t/domains-classes.t            |    2 
 3 files changed, 551 insertions(+), 3 deletions(-)