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

Allow compiler to select an strerror_r stringify

On Alpine (and potentially other systems) that don't identify their runtime correctly there is an issue with the string conversion
Specifically, alpine linux and musl where the errno_to_string is incorrectly called.
To fix this I have added two overloaded functions and use auto err to allow the compiler to detect the actual types returned and call the correct method for conversion

android sink - add retry mechanism

- in some cases subsequent calls to __android_log_write() may result with -EAGAIN error code.
  in such cases spdlog will sleep and try again for number of times defined by
  SPDLOG_ANDROID_LOG_NUM_OF_RETRIES

- defeult SPDLOG_ANDROID_LOG_NUM_OF_RETRIES set to 2 - can be overridden at build time

Add an optional final qualifier to types

When building with GCC's -Wfinal-types, a lot of types of spdlog
are marked as being more optimizable if they were marked final.

This patch adds a possibility for the user of the library to `#define SPDLOG_FINAL final`
and enjoy potentially better performance : GCC is then able to replace virtual calls by true
function calls if it can ensure that there are no derived types).

By default SPDLOG_FINAL is defined to nothing to not break existing code that
may be inheriting of some of these types for some reason.