AssertLog instead of Assert in some places

Hi All,
PostgreSQL code uses Assert() as a way to
1. document the assumption or conditions which must be true at a given
place in code
2. make sure that some bug elsewhere does not break these assumptions or rules
3. conditions which can not be (easily) induced by user actions

E.g. following conditions in adjust_appendrel_attrs()
/* If there's nothing to adjust, don't call this function. */
Assert(nappinfos >= 1 && appinfos != NULL);

/* Should never be translating a Query tree. */
Assert(node == NULL || !IsA(node, Query));

These conditions do not make it to non-assert builds and thus do not
make it to the production binary. That saves some CPU cycles.

When an Assert fails, and it fails quite a lot for developers, the
Postgres backend that caused the Assert is Aborted, restarting the
server. So a developer testing code that caused the Assert has to
start a psql again, run any session setup before running the faulting
query, gdb needs to be reattached to the new backend process. That's
some waste of time and frustration, esp. when the Assert does not
damage the backend itself e.g. by corrupting memory.

Most of the Asserts are recoverable by rolling back the transaction
without crashing the backend. So an elog(ERROR, ) is enough. But just
by themselves elogs are compiled into non-debug binary and the
condition check can waste CPU cycles esp. conditions are mostly true
like the ones we use in Assert.

Attached patch combines Assert and elog(ERROR, ) so that when an
Assert is triggered in assert-enabled binary, it will throw an error
while keeping the backend intact. Thus it does not affect gdb session
or psql session. These elogs do not make their way to non-assert
binary so do not make it to production like Assert.

I have been using AssertLog for my work for some time. It is quite
convenient. With AssertLog I get
```
#explain (summary on) select * from a, b, c where a.c1 = b.c1 and b.c1
= c.c1 and a.c2 < b.c2 and a.c3 + b.c3 < c.c3;
ERROR:  failed Assert("equal(child_rinfo, adjust_appendrel_attrs(root,
(Node *) rinfo, nappinfos, appinfos))"), File: "relnode.c", Line: 997,
PID: 568088
```
instead of
```
#explain (summary on) select * from a, b, c where a.c1 = b.c1 and b.c1
= c.c1 and a.c2 < b.c2 and a.c3 + b.c3 < c.c3;
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.
@!#\q
```

If there is an interest in accepting the patch, I will add it to the
commitfest and work on it further.

--
Best Wishes,
Ashutosh Bapat