Fix race condition starting CacheCleanupThread (#586)

This fixes two race conditions

1. Thread execution may start before `CacheCleanupThread.new` returns the class instance. In this case `t` is not yet defined inside the thread block and results in a "undefined method `sleepy_run' for nil:NilClass" error.

2. Thread execution may start before member variables `@store`, `@interval`, etc are initialized in `CacheCleanupThread#initialize` This results in "undefined method '*' for nil:NilClass" in should_cleanup? (this error only occurs after the first race condition is fixed)

More detail: A subclassed thread is scheduled for execution as soon as `super` is called. The code block of the thread may start to execute before the class is fully instantiated. See https://www.ruby-forum.com/t/thread-super-should-be-first-line-or-last-line/150617

lib/mini_profiler/storage/memory_store.rb

@@ -10,11 +10,11 @@ class MemoryStore < AbstractStore
       class CacheCleanupThread < Thread
 
         def initialize(interval, cycle, store)
-          super
           @store       = store
           @interval    = interval
           @cycle       = cycle
           @cycle_count = 1
+          super
         end
 
         def should_cleanup?
@@ -78,7 +78,7 @@ def initialize_cleanup_thread(args = {})
         cleanup_cycle    = args.fetch(:cleanup_cycle)    { CLEANUP_CYCLE }
         t = CacheCleanupThread.new(cleanup_interval, cleanup_cycle, self) do
           until Thread.current[:should_exit] do
-            t.sleepy_run
+            Thread.current.sleepy_run
           end
         end
         at_exit { t[:should_exit] = true }