Add the chain package and its buckets: builds, rollouts and assignments with history 64 (issue 1089)

internal/chain holds typed Build, Rollout and Assignment and a Backend with no delete or purge, in memory and on the bus. The builds and rollouts buckets are asserted with history 64, no age and values up to just under a mebibyte; the assignments bucket gets the same, keeping its marker age because a stream cannot be made to disallow message ages. Migration 0104 makes the one-row chain_moved switch. The controller keeps no consolidated schema, so 0104 is the whole schema change. Nothing reads the buckets yet.

internal/recordmirror/readers_test.go

@@ -20,6 +20,9 @@ import (
 // (internal/catalogue/state.go), and each mirror's own writer (this package and the loop that runs it). The
 // audit of a mirror against its store (ADR 0289 §4, issue 419) joins this list when it is built.
 var mayNameAMirror = []string{
+	// The chain opens its buckets, the assignments' among them, which stopped being a mirror when the chain took them
+	// (ADR 0408 §3, issue 1089).
+	"internal/chain/bus.go",
 	"cmd/mesh-controller/record_mirrors.go",
 	"internal/broker/controller_buckets.go",
 	// The grant of a machine's own keys of the assignments mirror to its provisioner client (design 57 P3).