Liveslots fails with virtual watchers in watchPromise

[mhofman]

Describe the bug

Providing a virtual watcher to watchPromise is asserted to be supported but will in fact fail because storing a virtual reference in a durable collection will throw.

agoric-sdk/packages/swingset-liveslots/src/watchedPromises.js

Lines 202 to 205 in 459998d

	virtual ||
	durable ||
	// separate line so easy to breakpoint on
	Fail`promise watcher must be a virtual object`;

agoric-sdk/packages/swingset-liveslots/src/watchedPromises.js

Line 225 in 459998d

watchedPromiseTable.init(vpid, harden([[watcher, ...args]]));

agoric-sdk/packages/swingset-liveslots/src/watchedPromises.js

Line 83 in 459998d

watchedPromiseTable = makeScalarBigMapStore('watchedPromises', {

To Reproduce

No test coverage, derived from auditing the code

Expected behavior

A virtual watcher should indeed be supported, but the collection used should be a virtual one, recreated for each incarnation.

Additional context

The watcher wouldn't be invoked in future incarnations, but that is not worse than a heap E.when that this is substituting. With a virtual watcher the program could avoid keeping the watcher in the heap.

See #9321 (comment)_