increase size of pg_commit_ts buffers

I wrote this patch last year in response to a customer issue and I
thought I had submitted it here, but evidently I didn't.  So here it is.

The short story is: in commit 5364b357fb11 we increased the size of
pg_commit (née pg_clog) but we didn't increase the size of pg_commit_ts
to match.  When commit_ts is in use, this can lead to significant buffer
thrashing and thus poor performance.

Since commit_ts entries are larger than pg_commit, my proposed patch uses
twice as many buffers.

Suffice it to say once we did this the customer problem went away.

(Andrey Borodin already has a patch to change the behavior for
multixact, which is something we should perhaps also do.  I now notice
that they're also reporting a bug in that thread ... sigh)

-- 
Álvaro Herrera                            39°49'30"S 73°17'W
"The problem with the future is that it keeps turning into the present"
(Hobbes)