rpc: pass errors from async_accept

According to the [boost::asio documentation](http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/reference/basic_socket_acceptor/async_accept/overload2.html),
the function signature of the handler must be:

    void handler(
      const boost::system::error_code& error // Result of operation.
    );

We were binding *all* the arguments, instead of all but the error,
resulting in nullary function that never got the error. Fix this
by adding an input argument substitution.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 0a0cd34
rpc: Make sure conn object is always cleaned up

Make sure conn object always gets cleaned up by using a
`boost::shared_ptr`.

This makes valgrind happy - before this commit, one connection object
always leaked at shutdown, as well as can avoid other leaks, when
for example an exception happens.

Also add an explicit Close() to the !ClientAllowed path to make it similar
to the normal path (I'm not sure whether it is needed, but it
can't hurt).

Rebased-By: Wladimir J. van der Laan
Rebased-From: 1a44522
doc: remove mention of `-rpctimeout` from man page

That option hasn't existed for a long time.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 381b25d
rpc: keep track of acceptors, and cancel them in StopRPCThreads

Fixes #4156.

The problem is that the boost::asio::io_service destructor
waits for the acceptors to finish (on windows, and boost 1.55).

Fix this by keeping track of the acceptors and cancelling them before
stopping the event loops.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: cef4494