names: turn registry lock into rwsem

names: turn registry lock into rwsem

Imagine a racing name-acquire and msg-send. The msg-send might lookup the
destination name, which might be an activator. But before it can queue the
message, the racing name-acquire installs and implementor and moves all
pending messages from the activator to the implementor. If the original
msg-send afterwards continues, it will still queue the in-flight message
on the activator instead of the implementor.

After 1 day of discussion, we decided on turning the registry into a
read-write lock. This way, any msg-send will be locked against
name-changes. A name-change requires a writer lock, but any in-flight
message will hold a regsitry read-lock from the name-lookup until it
queued the message.

This patch only converts the lock, following patches will close the races.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>