Make trantor::Logger::setOutputFunction thread safe

[Demilivor]

The call to the trantor::Logger::setOutputFunction is not thread safe:

trantor/trantor/utils/Logger.h

Lines 124 to 139 in 8bf280b

	static void setOutputFunction(
	std::function<void(const char *msg, const uint64_t len)> outputFunc,
	std::function<void()> flushFunc,
	int index = -1)
	{
	if (index < 0)
	{
	outputFunc_() = outputFunc;
	flushFunc_() = flushFunc;
	}
	else
	{
	outputFunc_(index) = outputFunc;
	flushFunc_(index) = flushFunc;
	}
	}

2 threads can simultaneously set the values, or at least 1 simulaneous reader can read the value during the change of output function.

Proposal solution:
Use std::shared_mutex here. Use exclusive lock during modification and shared lock during the reading. This sure will consume some performance, but at least will be thread-safe.