Add schema versioning to the database

[ketiltrout]

Even if we didn't support multiple schema versions in a single alpenhorn version, we could at least have an out-of-date alpenhorn complain and exit rather than screw up the database.

The proposal here: chime-experiment/Infrastructure#21:

I'd also recommend adding a table for schema versioning, say schema_version (Or schemaversion since we seem to like smashing words together. Is that what peewee wants?). Given there are multiple extensions in alpenhorn2, maybe have two columns:

   component CHAR(60) NOT NULL PRIMARY KEY
   version INT NOT NULL

and then have at least one column for the alpenhorn2 base schema and maybe one for the CHIME extensions:

component	version
alpenhorn	1
chime	1

Does it need a peewee model? I suppose, if we're using peewee's migrations per #54 )