In async mode, teardown callback on worker thread exit

(Note: this can be helpful when a custom sink invokes a JNI callback, which implies that worker thread was previously attached to JVM, and needs to be cleanly detached upon exit)

Revert "CMake script improvements"

CMake script improvements

It's no longer a standalone cmake file because cmake was crashing when
reading it as part of the subproject.

Automatically flush log if message level is above certain severity.

daily_file_sink with custom file name calculator

update comments

Generate & Install pkg-config File

Performance improvement of stdout sink 

Add the SPDLOG_USE_WCHAR tweak to enable support for Unicode names on Windows. Refs #111