TypedMessageQueue: the computing of "pending_" was prone to race conditions and is now performed from pop_front
AsyncWriterSource: EPOLLOUT would be ignored after processing events from the message queue
Runner: fixed the case where the closing of an std output channel would occur
twice, leading to a SEGFAULT after accessing a nil sink ptr
Runner: simplified the handling of file descriptors by not using EPOLLONESHOT
async_writer_source: missing #pragma once