Fixes #324, #325, and #402

This PR adds a set of callbacks that can be used to track the lifecycle of TURN allocations in the server. This allows user code to be called whenever an authentication request has been processed (this can be used to mitigate DoS attacks against a server, see #402), when an allocation has been created/removed (e.g., to implement user tracking for adding quota support, see #324), when a permission has been created/removed (e.g., to log the peers a user tries to reach via the server), a channel has been created/removed (this will simplify integrating external TURN accelerators, see #360), or when an allocation ends with an error.

The implementation adds a new EventHandlers struct to the ServerConfig, defined as follows:

type EventHandlers struct {
  OnAuth func(srcAddr, dstAddr net.Addr, protocol, username, realm string, method string, verdict bool)
  OnAllocationCreated func(srcAddr, dstAddr net.Addr, protocol, username, realm string, requestedPort int)
  OnAllocationDeleted func(srcAddr, dstAddr net.Addr, protocol, username, realm string)
  OnAllocationError func(srcAddr, dstAddr net.Addr, protocol, message string)
  OnPermissionCreated func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.IP)
  OnPermissionDeleted func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.IP)
  OnChannelCreated func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.Addr, channelNumber uint16)
  OnChannelDeleted func(srcAddr, dstAddr net.Addr, protocol, username, realm string, peer net.Addr, channelNumber uint16)
}

It is OK to handle only a subset of the events.

Caveats:

• Event handlers are called synchronously, so long-running callbacks will block the server.
• Theoretically, allocations are allowed to change credentials during their lifetime, in that any Refresh, CreatePermission or ChannelBind message may use a different credential. Since this does not seem to be easy to implement with the PeerConnection API, credential swaps are currently no tracked.