Rework connection teardown logic

[rblenkinsopp]

This makes a number oc changes to improve Connection teardown logic:

• Fixed a file descriptor leak in Connection that exhausted the Unity process's OS handle limit after repeated play-mode start/stop cycles.
• Fixed an unmanaged memory leak where the marshalled server namespace string passed to LeapC was not freed.
• Fixed a managed memory leak in Controller where event subscriptions on the pooled Connection were not removed on disposal.
• Fixed a missing timeout on Connection shutdown which could lead to delays in applications exiting.