[bug] fix the couch schema migration script
So we foud that some databases contained data from old migrations that were never deleted from the database, and that caused the script to blow.
I have added some modifications:
- account for leftover documents, delete them.
- avoid failing when encountering remainings of a previous interrupted migration, if the content matches what is expected.
- improved logging (have the db name in every line, log exceptions, include verbose option).
- skip one db in case of error, continue with next one.