LockObject patch

Hackers,

Here is the LockObject patch I was able to come up with. It's almost
the same patch that Rod Taylor published two years ago; basically, it
expands LOCKTAG with a ClassId attribute, and provides a LockObject
method to allow locking arbitrary objects. I omitted the uses of the
new functionality.

I provide this mostly for review purposes; it's not intended to be
applied, because I'll submit it in conjunction with the shared
dependency patch later.

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"El sabio habla porque tiene algo que decir;
el tonto, porque tiene que decir algo" (Platon).