fix #970: adjust dbpatch and password tables

The constraint on the `dbpatch` table is adjusted so that, while attempts to repeat the same DB upgrade are still prevented, the user may switch between different server branches easily enough even if they have different revisions of Bio-Formats, the server starting each time without trouble.

The `password` table acquires a new `changed` column, noting the latest time that a user's password was set. First, try a server that includes #3061 but not this PR: change some users' passwords, including some more than once. Now, run the upgrade script and ensure that the `changed` column of the `password` table contains correct times for when those passwords were most recently changed. (Times for `root` might be a bit newer than seems correct, as that user is a bit odd.) Finally, start a server with this PR included, and make sure that further password changes, and passwords of newly created users, continue to have appropriate times updated in this column.

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/970.
--no-rebase