winery: merge databases into a single database managed by `swh db`

(stacked on top of !183 (merged) and !184 (merged), only the last three commits to review)

Instead of winery managing multiple databases by itself, creating quite a complex database management lifecycle, simplify things drastically by using the common SWH database management scaffolding.

Instead of separate databases, read-write shards now use separate tables within the same database, created from the shard_template template table. The throttler tables are also merged into the main database.

We completely drop the shard_dsn argument to all winery components.