LU-1522 recovery: rework LU-1166 patch in different way

LU-1522 recovery: rework LU-1166 patch in different way

Dropping recovery counters upon last export put caused LU-1522 issue,
return class_export_recovery_cleanup() back to the
class_export_disconnect() and use exp_failed flag to avoid race
between target_handle_connect() and class_disconnect_stale_exports()

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: I78c19a8d49786877d2de27c82bf40ebec494f044
Reviewed-on: http://review.whamcloud.com/3145
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>