When the cached_fs_rom saturates the packet stream of its File_system
session it will call the session request handler recursively as pending
transfers are completed. This is bad because the content of the XML node
currently being processed will change.
The session request handler can no longer be called directly, but the
"schedule" method will submit a signal to the request handler, and
requests will be processed after the current operation has completed.
This component is contrasted with the fs_rom server that serves
independent dataspaces to each client. Using a cache was not possible
until the region map session supported the creation of read-only
attachments.
Test at run/read_only_rom.
Ref #1633Fix#2760
