sinval.c / sinvaladt.c restructuring

I just modified the interactions in sinval.c and sinvaladt.c per
http://thread.gmane.org/gmane.comp.db.postgresql.devel.patches/18820/focus=18822

It looks a lot saner this way: the code that actually deals with the
queue, including locking etc, is all in sinvaladt.c.  This means that
the struct definition of the queue, and the queue pointer, are now
internal "implementation details" inside sinvaladt.c.

One side effect of this change is that the call to SendPostmasterSignal
now occurs after the lock has been released.  ISTM this is a good idea
on general principles (no syscalls in lwlocked code), but I'm wondering
if I created a thundering hoard problem that did not exist before.

All tests pass.

As a test of Review Board, I uploaded the patch to it:
http://reviewdemo.postgresql.org/r/19/

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.