isLocalLeader has race condition with mastership check.

Mastership state can persist for some time on master during device disconnection.
issue is that onos-2 has
[90m12:21:26.574[0m [1;32mINFO [m [90m[Olt][0m master Checking master for: of:00000a0a0a0a0a00
[90m12:21:26.574[0m [1;32mINFO [m [90m[TopologyManager][0m Topology DefaultTopology

{time=1040640246834531, creationTime=1620217286560, computeCost=207179, clusters=0, devices=0, links=0}

changed
[90m12:21:26.574[0m [1;32mINFO [m [90m[Olt][0m master Local master. Returning true.
[90m12:21:26.574[0m [36mDEBUG[m [90m[Olt][0m OLT got DEVICE_AVAILABILITY_CHANGED event for DefaultDevice

{id=of:00000a0a0a0a0a00, type=SWITCH, manufacturer=VOLTHA Project, hwVersion=open_pon, swVersion=open_pon, serialNumber=BBSIM_OLT_0, driver=voltha}

/null
[90m12:21:26.575[0m [1;32mINFO [m [90m[Olt][0m Handling controlled device disconnection .. flushing all state for dev:of:00000a0a0a0a0a00
because it was the master of that device
but also onos-1
does the same operation
[90m12:21:26.601[0m [1;32mINFO [m [90m[Olt][0m master Checking master for: of:00000a0a0a0a0a00
[90m12:21:26.601[0m [1;32mINFO [m [90m[Olt][0m master Not master for device: of:00000a0a0a0a0a00
[90m12:21:26.613[0m [1;32mINFO [m [90m[Olt][0m master Checked is local leader. Returning true.
[90m12:21:26.613[0m [36mDEBUG[m [90m[Olt][0m OLT got DEVICE_AVAILABILITY_CHANGED event for DefaultDevice

{id=of:00000a0a0a0a0a00, type=SWITCH, manufacturer=VOLTHA Project, hwVersion=open_pon, swVersion=open_pon, serialNumber=BBSIM_OLT_0, driver=voltha}

/null
[90m12:21:26.614[0m [1;32mINFO [m [90m[Olt][0m Handling controlled device disconnection .. flushing all state for dev:of:00000a0a0a0a0a00