SysOTA

The previous approach for making reboot pending postpone service
shutdown was not robust, because it would only extend the interval of
the check, not stop the actual exit.

It worked fine for shorter periods, as the default (60 second) value was
masking what the integration tests were measuring (10 seconds).

Extend RebootDelayer interface with a new proto.RebootPending method
which checks if a reboot timer exists. This makes the check race-free
regardless of the frequency.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>