Close pipe by ID

[gdamore]

For messages, we could store the PIPE ID as a message property. This could then be used by the application to conditionally close the underlying pipe (if it still exists). Using IDs here would solve questions about use-after-free, and avoid needing to hold any locks or long term reference counts.

[gdamore]

nng_msg_get_pipe() already exists now.
nng_pipe_close() exists too; unfortunately it uses a pointer to nng_pipe, instead of an ID like sockets and endpoints. We need to fix this.

[gdamore]

This is fixed now, but we need a test for it.

[gdamore]

The pipe_close_test branch validates this -- but we can confirm that this is now working at any rate.