Evict archive writers on deletion and sweep idle archives (closes #89) (#95)

Per-webhook archive writers are now evicted when the webhook or its last
database target is deleted, and a background sweeper prunes expired rows from
idle archives that no longer receive writes. Archive files themselves are never
deleted.

internal/delivery/target_database_test.go

@@ -50,6 +50,13 @@ func openArchiveDBForRead(
 	return gdb
 }
 
+// archiveFileSuffixes returns the archive file itself and the
+// SQLite sidecars that accompany an open database. A test that
+// asserts no archive was created has to check all of them.
+func archiveFileSuffixes() []string {
+	return []string{"", "-wal", "-shm"}
+}
+
 // removeArchiveFiles simulates an operator moving the archive
 // away by deleting the SQLite file and its sidecar files.
 func removeArchiveFiles(t *testing.T, path string) {