backend: Implement next_ops->reopen call.

backend: Implement next_ops->reopen call.

This is intended for use by the forthcoming retry filter to close and
reopen the backend chain.  It is handled entirely by server/backend.c
as no cooperation is needed with the plugin.

Note the explicit readonly parameter: An alternative would be to store
the previous readonly setting in the b_conn_handle struct.  However
passing it explicitly allows the retry filter to retry as readonly,
which might be useful.  This design does however require any filter
which might call .reopen to save the original readonly parameter from
the .open call.