chore: capture more informative stack trace in session leak checker (#2618)
chore: capture more informative stack trace in session leak checker (#2618)

Since we migrated to acquiring a server session lazily the stack
traces for the sesson leak checker haven't been very helpful. This
patch records the trace from the call to `Topology#startSession` in
order to provide a more high quality trace.

NODE-2880
mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native