spacemanager: Maintain aggregate columns with database triggers

Moves the logic to maintain aggregate columns in srmlinkgroup and srmspace from
Java code to database triggers. Adds not-null constraints to several space
manager database columns. This may take a little while to apply on large
databases.

The schema modifications are not compatible with older versions. Should a
downgrade be needed, the schema modifications have to be rolled back using
'dcache database rollback <tag> SrmSpaceManager@<domain>' or 'dcache database
rollback <date> SrmSpaceManager@<domain>'. The former requires that a tag was
created before upgrade (this is recommended). Should the system be downgraded
without rolling back the schema changes, the space accounting will be
incorrect.

Target: master
Require-notes: yes
Require-book: yes
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6370/