v.io/x/devtools/vmon: use lock to protect concurrent accesses to map
v.io/x/devtools/vmon: use lock to protect concurrent accesses to map

The map "aggs" was being modified concurrently by the statsWorker()
threads.  This caused crash:
	https://veyron.corp.google.com/jenkins/job/monitoring-cloud-syncbase/109367/console

This change introduces a mutex to fix it.

Change-Id: I1530c68131af162499ea97802efa32417a647266

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools