veyron/runtimes/google/ipc: fixing a deadlock in the context code
veyron/runtimes/google/ipc: fixing a deadlock in the context code

Prior to this CL, the following sequence of calls can occur resulting
in a deadlock:

- parent context that has been cancelled receives addChild(child)

- this triggers child.cancel() while holding parent.mu

- this in turns triggers parent.removeChild(child), which requires
  parent.mu

This CL fixes this issue by releasing the parent.mu before invoking
child.cancel().

Change-Id: I0fd58e57bda5809e587d287059c547a4ea114f80

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref