restore: TASK_HELPERs live until RESTORE stage ends

In order to use TASK_HELPERS to open files from dead processes, they should
persist until criu is done restoring the filesystem, which happens in the
RESTORE stage. To do this, we need to pass each helper's PIDs to the restorer
blob, so that it can wait() on them when the restore stage is done.

This commit is in preparation for the remap_dead_pid commits.

v2: wait() on helpers after restore stage is over
v3: add CR_STATE_RESTORE_FS stage
v4: CR_STATE_RESTORE_FS waits for nr_tasks + nr_helpers, not nr_threads
v5: ditch CR_STATE_RESTORE_FS in favor of passing helpers to restorer blob

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>