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.
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).
doc: remove mention of `-rpctimeout` from man page

That option hasn't existed for a long time.
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.
Merge pull request #4142

cef4494 rpc: keep track of acceptors, and cancel them in StopRPCThreads (Wladimir J. van der Laan)
381b25d doc: remove mention of `-rpctimeout` from man page (Wladimir J. van der Laan)
1a44522 rpc: Make sure conn object is always cleaned up (Wladimir J. van der Laan)
0a0cd34 rpc: pass errors from async_accept (Wladimir J. van der Laan)