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
lock renames run under, 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 lock, 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,