rxrpc: Allow the reply time to be obtained on a client call

Allow the epoch value to be queried on a server connection.  This is in the
rxrpc header of every packet for use in routing and is derived from the
client's state.  It's also not supposed to change unless the client gets
restarted.

AFS can make use of this information to deduce whether a fileserver has
been restarted because the fileserver makes client calls to the filesystem
driver's cache manager to send notifications (ie. callback breaks) about
conflicting changes from other clients.  These convey the fileserver's own
epoch value back to the filesystem.

Signed-off-by: David Howells <dhowells@redhat.com>