insert ONU interface lock thread before MIB download

build_dt-berlin-pod-openonugo_1T8GEM_voltha_DT_openonugo_master_test runs into error TC 'Test Disable ONUs and OLT Then Delete ONUs and OLT for DT' with Sercom ONU, which obviously needs disable/enable toggling here to allow traffic. This is how the py-adapter works per default (which does not show the problem).

Problem may not appear with other ONU types (e.g. xgspon alpa, Adtran ONU test outstanding), but nevertheless the workaround is applied generally. Maybe it is useful also later in respect to better ONU interface state notifications as mentioned in the py-code:

• # start by locking all the unis till mib sync and initial mib is downloaded
• # this way we can capture the port down/up events when we are ready