[2d] Allow 'mount-snapshot' events in many more state machine handlers

[binocarlos]

User requirement

As a user of dotscience, I want to be able to view any previous snapshot so that even if a transition is happening to a later snapshot - I can still view files in the snapshot I am interested in

This happens for example when trying to view files whilst a dot is in 'pushPeerState' - we are unable to mount a read only snapshot because it's only the active state that handles the mount-snapshot event.

This will break many things for dotscience whilst large files are being uploaded from the runner

Key acceptance criteria

• It will allow the mounting of snapshots when transfers are taking place so the user can still see the list of files