Worker1: use a list, rather than a single slot, to manage the default db link so that we can keep the least-recently-opened db as the default. Re-introduce worker1.export() but do not yet expose it until an equivalent import() (or open() option) is implemented.

FossilOrigin-Name: c7750c101d9b7baa31496731bc34ea0a9b2bad0c11e2b3e92a8c7b327135c1bb