ConnectionManager: refactor to reduce contention

* merge info, waiting and pending maps into DeviceInfo,
  improving consistency of various operations
* DeviceInfo has its own mutex,
  and operations keep a weak pointer to it,
  reducing contention
* DeviceInfoSet encapsulate atomic management of DeviceInfos,
  enforcing proper locking order

Change-Id: I12c107782600355a34d460e7802d92ae9d590993