offset thread ids where possible (#130)

- configure known background threads to start indexing down from TIMEMORY_MAX_THREADS
- invoke sampling::shutdown() instead of just blocking signals where possible

source/lib/omnitrace/library/process_sampler.cpp

@@ -74,10 +74,11 @@ get_sampler_is_sampling()
 void
 sampler::poll(std::atomic<State>* _state, nsec_t _interval, promise_t* _ready)
 {
-    OMNITRACE_SCOPED_THREAD_STATE(ThreadState::Internal);
-
+    threading::offset_this_id(true);
     threading::set_thread_name("omni.sampler");
 
+    OMNITRACE_SCOPED_THREAD_STATE(ThreadState::Internal);
+
     // notify thread started
     if(_ready) _ready->set_value();