Create the session tracker after configure has run

Create the session tracker if sessions are enabled to avoid the overhead
of creating it on the first request

By requiring the concurrent gem in the session tracker's initialize
method, we shifted the overhead from startup to the first request.
This caused the first request to go from ~10ms to ~70ms (in the Sinatra
example app run locally), which is quite alot of overhead

lib/bugsnag.rb

@@ -48,6 +48,12 @@ class << self
     def configure(validate_api_key=true)
       yield(configuration) if block_given?
 
+      # Create the session tracker if sessions are enabled to avoid the overhead
+      # of creating it on the first request. We skip this if we're not validating
+      # the API key as we use this internally before the user's configure block
+      # has run, so we don't know if sessions are enabled yet.
+      session_tracker if validate_api_key && configuration.auto_capture_sessions
+
       check_key_valid if validate_api_key
       check_endpoint_setup