Instrument sync_array waits with tpool_wait_begin/end

instrument sync_array waits with tpool::wait_begin/end

threadpool has to know about long waits, to be able to create or wakeup new threads, or it can become slow, if it just relies on maintainence timer.

As an example, asynchronous read notification can stuck in the threadpool queue for a while,
if many threads are waiting for it, and threadpool is not notified.