remus: don't do failover if we don't have a consistent state

remus: don't do failover if we don't have a consistent state

We will have a consistent state when a CHECKPOINT_END record
is received. After the first CHECKPOINT_END record is received,
we will buffer all records until the next CHECKPOINT_END record
is received. So if the checkpoint() callback returns XGR_CHECKPOINT_FAILOVER,
we only can do failover if ctx->restore.buffer_all_records is
true.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>