libctdb: change callback for ctdb_readrecordlock.

libctdb: change callback for ctdb_readrecordlock.

After discussion with Ronnie, we decided to revisit this interface.  We use
the name ctdb_readrecordlock_async, as it is *not* always a send, and we
use a specific callback to avoid the "fake request" creation on the fast
path.

The request itself is never exposed: this means it can't be cancelled,
but we can revisit that later if need be.

This makes both use and implementation simpler.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>