spacemanager: Fix null constraints and other schema migration issues

The old space manager allowed null values for several fields. According
to the code, these fields should not actually become null, however at
least one site reported seeing records with null values. Either this
is from an undetected bug in the old code or because older versions of
dCache created records with null values. Such null values cause the schema
migration to fail. This patch resolves the issue by adding a default value
for some of the null fields.

For the remanining fields that could be null in the schema but which shouldn't
be null, the patch adds an explicit check to instruct sites to contact us.

The patch also adds a check for site local computed indexes on expiration
time - at least one site had this.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Require-notes: yes
Require-book: no
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8567
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7579/
Merge pull request #1166 from gbehrmann/fix/2.8/rb7579

spacemanager: Fix null constraints and other schema migration issues