jobs: Go back to the event-driven model to send logs

jobs: Go back to the event-driven model to send logs

When the builder is not located in the same data center it can happen
that the send buffer fills up which results in a broken TCP connection
if we continue to send any data.

We will now first of all buffer any messages that have only been
partially sent and listen for when the socket is ready to receive the
rest. If so, we will flush our buffer.

Afterwards we will call the send callback which will go and look for any
more data to send.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>