Fix race in unix socket transport.
Fix race in unix socket transport.

transport.baseEndpoint.receiver and transport.baseEndpoint.connected are
protected by transport.baseEndpoint.Mutex. In order to access them without
holding the mutex, we must make a copy.

Notifications must be sent without holding the mutex, so we need the values
without holding the mutex.

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor