Merge remote-tracking branch 'origin/thrift_async'
Merge branch 'master' of github.com:apohl79/libtasks
Reimplemented async thrift over uwsgi support

Instead of using threads to enable async thrift handling, the async
processor handles thrift messages now internally.

Executor thread pool

A new thread pool has been added that enables async operations similar
to std::async but threads will be reused. The thread pool grows and
shrinks dynamically. See test_exec.cpp for examples.

Task refacturing

Introduced the event_task and moved libev specifics from task to this
new class.