First cut on asynchronous append using thread pool.
Update ThreadPool.
Merge branch master into master-async-append.
Update threadpool. Wait until no more events are being logged asynchronously.
Update ThreadPool.
Wait for async logging to finish on hierarchy shutdown.
Bump SO version.
Update ThreadPool.
Update ThreadPool.
Fix single threaded builds.
Use nullptr more.
patternlayout.cxx: Add default LiteralPatternConverter ctor to construct empty
string literal.
Use internal::empty_str instead of "" as empty string argument.
timehelper.cxx: Use braced initializer for table members.
Merge pull request #64 from wilx/master

Merge async logging and some other improvements.