Move the archive files into their own package (closes #541)

The SQLite archive files the database target writes are now managed by
internal/archive: file names, rotation, the writer of each target,
pruning, renames, the gzipped JSON export, the idle sweep, and the
rename lock, which was Handlers.renameMu. The database target stays in
internal/delivery and writes through archive.Writers; the engine still
closes the writers when it stops, so the stop order is unchanged. The
handlers take the rename lock from archive.Archives.RenameLock, and
rename and evict through archive.Archives.

Exported names drop their Archive prefix in the new package. The tests
moved with the code; the two that check a delivery's recorded attempt
stay in delivery.

Model: opus-5-5

internal/db/migrations/archive/001_schema.sql

@@ -1,5 +1,5 @@
 -- Migration 001: the schema of a database target's archive file. The
--- archivedEvent model in internal/delivery must agree with it.
+-- archivedEvent model in internal/archive must agree with it.
 
 CREATE TABLE archived_events (
     id integer PRIMARY KEY AUTOINCREMENT,