core: Save schema version for intermediate steps

core: Save schema version for intermediate steps

When running upgrade queries, update the schema version for each
intermediate step.  This ensures that any interrupted upgrades have a
greater chance of resuming correctly after core restart.

Almost all databases make single queries atomic (fully works or fully
fails, no partial), and with many of the longest migrations being a
single query, this makes upgrade interruptions much more likely to
leave the database in a valid intermediate schema version.

As a side effect, downgrading from a partial upgrade will now require
setting schema version manually, instead of it possibly happening to
work depending on what changed.  An unsupported operation failing in
a known, recoverable way seems better than failing in an unknown,
possibly recoverable way.

Update logging to be more explicit about what fails.

Tested by kill -9'ng Quasselcore during migration, then re-running
core afterwards.

In the future, for databases that support it (e.g. almost only
PostgreSQL), we may want to wrap upgrades in a transaction.  This
will need careful testing of potential additional space requirements
and any database modifications that might not be allowed in a
transaction.

sqlite> select * from coreinfo;
schemaversion|26
$ (sleep 0.5 && pkill -9 quasselcore ) & \
  ./run-profile.sh master core local
sqlite> select * from coreinfo;
schemaversion|30
$ ./run-profile.sh master core local
sqlite> select * from coreinfo;
schemaversion|31