tor-keymgr: put ephemeral keystore behind experimental "ephemeral-keystore" feature
Followup of !2424 (comment 3078939), part of #1623.
This puts ArtiEphemeralKeystore
behind a new experimental "ephemeral-keystore" tor-keymgr feature.
This will slightly conflict with !2411 (merged) which adds ArtiEphemeralKeystore
to TorRelay
.