fix deadlock when refreshing probes. fixes #86

[woodsaj]

When a probe is removed while the socketCache refreshLoop is iterating over probeSessions a deadlock was being reached.

The deadlock was reached when the following happened:

• the refreshLoop obtains a RLock on the socketCache
• a probe disconnected and probe.OnDisconnection() is called
• probe.OnDisconnection() obtains a writeLock, then calls socketCache.Remove()
• socketCache.Remove tries to obtain a writeLock but can't because the refreshLoop has a RLock
• the refreshLoop calls probe.LastRefresh() on the probe that just disconnected
• probe.LastRefresh() tries to obtain an RLock but cant because probe.OnDisconnect still has a write Lock.
• DEADLOCK