Bluetooth: Allow disconnected state connections to be reconnected

Make it possible to initiate new connections from within the
disconnect callback. This wasn't completely trivial since there was
connection cleanup done through deferred action using the CONN_CLEANUP
flag.

This patch moves the disconnected callbacks to be run after all
cleanup is done. We can't directly do this in the TX thread, since
that's internal, so we instead take advantage of the deferred work
support and do it using the update_work callback. Since the same
cleanup is needed also for BR/EDR connections the work definition is
moved from the LE-specific struct to the generic struct bt_conn.

A valid bt_conn object in disconnected state is a likely indication of
a connection reference leak, so there's a new BT_WARN() for this case
in bt_conn_create_le().

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>