Ask again for a folder that moved, and carry entries left while a disk was away (hq ADR 0325)

The engine names the filesystem a waiting folder is on; a warrant is recorded for that filesystem, and a
folder on another is asked about again and not taken there meanwhile. Entries the engine could not take
out while their disk was away, stated before a send, do not hold that delivery's gate. mesh-host pinned
to d2d9fbbd.

internal/inventory/migrations/0096-a-module-is-given-an-identity-and-a-folder-is-taken.sql

@@ -25,7 +25,7 @@ create table module_identity (
 -- everything there, and the folder is taken: kept from then on without asking, a module that gains the access
 -- added with no wait. The row is the record of that act — who approved it, under which warrant — and what tells a
 -- later module's access that it joins a taken folder (`taken` on the access). The path is the folder as placed,
--- clean and absolute.
+-- clean and absolute. A new warrant for the same folder on another filesystem replaces the row.
 create table kept_folder (
     node     uuid        not null references node (id) on delete cascade,
     path     text        not null check (path like '/%'),
@@ -36,5 +36,9 @@ create table kept_folder (
     warrant  text        not null,
     -- What the node-engine answered when it started the take.
     said     text        not null default '',
+    -- The filesystem the folder was on when it was taken, as the engine names it without a path ("fs-<12 hex>"):
+    -- a wait naming another (the folder moved to another disk) is not answered by this take, and the operator is
+    -- asked again; until the new warrant the folder is not taken there. Empty from an engine that names none.
+    placed   text        not null default '',
     primary key (node, path)
 );