Allow user to define a printf()-like function
to be sued for logging; disable logging if not defined
esp-open-rtos implementation of printf()
generally can't handle uint64_t -- See Issue #561
use (long double) to represent the time error in ticks
The function sntp_set_servers() duplicates the strings supplied in the
server_url[] array into new strings but forgets to allocate the extra
byte needed for the \0 terminator for each string.
Fix the problem by using strdup(), which allocates the right amount of
memory and copies the string at once.
