Keep builds in the builds bucket and move the build rows there at start (issue 1090)

Once chain_moved records the kind, RecordBuild and every build read go through the chain (reuse key and published added); before that the tables are read as before. The serving controller copies the rows at its start, holding the lease: idempotent per key, rows counted against keys, the largest value measured, a refusal (urgent condition, nothing switched) on a count that differs or a value over the bus's largest, and a re-read when the table took a row meanwhile. ForTest attaches the chain in memory with builds moved; ForTestBefore leaves it unmoved. Build requests, gates and the other kinds are still in their tables.

internal/inventory/source.go

@@ -32,6 +32,11 @@ type sourceRow struct {
 // sourceRows is every successful build of each module (of one module, when named), newest request
 // first.
 func (i *Inventory) sourceRows(ctx context.Context, module string) (map[string][]sourceRow, []string, error) {
+	if c, on, err := i.ChainReads(ctx, KindBuilds); err != nil {
+		return nil, nil, err
+	} else if on {
+		return i.chainSourceRows(ctx, c, module)
+	}
 	query := `select b.module, b.id, b.commit_hash, b.source_fingerprint, b.made, b.manifest,
 	                 coalesce(g.verdict = 'failed', false)
 	            from build b left join build_gate g on g.build = b.id