Make db revision step numbers global

[Anton Gritsay]

Rational: sometimes the migration step crosses the boundaries of tablespaces. In such case it may become impossible to achieve with current db migration architecture both of two goals: 1) split migration scripts by tablespace (riski, oqmif...); 2) guarantee the order of execution of migration steps.

We may make revision step numbers global. And it would be better then to change directories structure - place step number at higher level and create tablespace directories inside.

[Mattia Barbon (mattia.barbon)]

Bumped to the same priority/milestone as https://bugs.launchpad.net/openquake/+bug/816454

[Mattia Barbon (mattia.barbon)]

I propose to change the directory structure to: upgrades/openquake/<version>/<step> and remove the schema subdirectory.

Rationale: having schema subdirectories gains us nothing if the change involves just one schema (the schema name can be in the file names) and is actively harmful if the change involves multiple schemas (the upgrade might require multiple changes to both schemas and we might need to interleave the scripts).

[Lars Butler (lars-butler)]

#2 LGTM

[Mattia Barbon (mattia.barbon)]

Pull request https://github.com/gem/openquake/pull/401