Bug in server status view

The server status view does not properly display the "blocked by"
field, unless it's blocked by a table level lock. In particular, if
you create a row-lock, it shows empty.

Attached patch changes two things:
1) Detects lock conflicts on transaction id's, which is where row
locks normally show up
2) Change it so that if we know the query is blocked, but we can't
figure out who it's blocked by, it shows -1. Today, it just shows
NULL, which is exactly the same as it shows if it is not blocked at
all - making it quite useless...

Comments?

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/