netlink-socket: Async notifications are incompatible with other operations.

netlink-socket: Async notifications are incompatible with other operations.

A Netlink socket that receives asynchronous notifications (e.g. from a
multicast group) cannot be used for transactions or dumps, because those
operations would discard asynchronous messages that arrive while waiting
for replies.

This commit documents this issue in a comment on nl_sock_join_mcgroup().
It also removes an internal attempt to avoid mixing multicast reception
with other operations.  The attempt was incomplete, because it only
handled dumps even though ordinary transactions are also problematic.  It
seems better to remove it than to fix it because, first, all of the
existing users in OVS already separate multicast reception from other
operations and, second, an upcoming commit will start using unicast
Netlink for asynchronous notifications, which has the same issues but
doesn't use nl_sock_join_mcgroup().