Fixed issues 449

Please check it

implement message counter feature

adds %i logger pattern for printing log message sequence ID

Fix OSX builds of details/os.h

Minor fixes

Revert "Merge pull request #441 from alzix/count_discarded"

This reverts commit 03873334, reversing
changes made to 862d2f6f.

add an option to warn about discarded messages

cover discarded messages use-case

fix - formatter new discarded message

add wide string to utf8 string support

when using async_logger with async_overflow_policy::discard_log_msg each discarded
message will be counted and warning will be printed by the worker thread

this new feature is disabled by default - as it may have a performance hit when discarding messages