Bug 3081:
During conversion of listening socket handlers to AsyncCalls a violation
of the AsyncCall API was introduced. Resulting in occasional crashes from
invalid re-use of call objects.
This implements a TcpAcceptor async job which receives a listening socket
and a CallSubscription. For every connection attempt on the listener socket
a new AsyncCall is spawned from the subscription template.
Initial users are the HTTP and HTTPS listening sockets and FTP data channel.
In order to implement this job in FTP the logics surrounding data channel
handling had to be extended and reworked. Fixing bug 2948 and 2581 in the
process.