Add performance counters to measure current connections by transport
Keep previous logic for updating ConnectionsCurrent PerfCounter.
Merge branch 'dev' of https://github.com/SignalR/SignalR into dev
Re-add connection counter methods for longPolling transport (merge conflict resolved)
Fix summary comments and descriptions in PerformanceCounterManager.
Merge pull request #2971 from thomaswr/dev

Add performance counters to measure current connections by transport
Handle transport failures occurring in the connecting state

- In the event of any transport error prior to the init message,
  the transport should immediately stop and allow the connection to
  fall back to another transport.
- In the event of any transport error during the start request,
  the entire connection should immediately be stopped