[Debug] implement logging to a memory-mapped log file

When setting the environment variable EMPER_LOG_FILE=<logfile> EMPER will write its log messages to <logfile> instead of stderr. This removes the need for the mutex protecting std::cerr as well as multiple write calls to flush std:cerr.

To efficiently write log messages to the log file the algorithm uses three memory mapped views into to store the log messages. One buffer is active, one is new, and one is old. The next buffer ensures that threads can write log messages even if the active buffer would overflows. The old buffer allows slower threads to still write safely while everyone else uses the active buffer. When a thread changes from the active buffer to the new buffer it is responsible to renew the current old buffer and changing the semantic of the buffers:

• active -> old
• next -> active
• old -> next

This buffer scheme allows wait-free logging. But the approach is NOT sound because delayed thread may still use the old buffer which gets renewed by the first thread touching the next buffer. But the likeliness for this situation decreases with bigger sizes of the buffers.

ATTENTION: Using SCHED_IDLE for the completer may break this likeliness assumption.

emper/io/GlobalIoContext.hpp

@@ -15,7 +15,7 @@ class Runtime;
 namespace emper::io {
 class GlobalIoContext : public IoContext {
 	friend Runtime;
-	friend void ::emper_log(const std::string& prefix, const std::string& message);
+	friend void emper::log::log(const std::string& prefix, const std::string& message);
 	friend class Future;
 	friend class SendFuture;
 	friend class RecvFuture;