…own on large numbers of requests.

Note that we create a destructor on the command to reset request->cmd
pointer if the cmd is freed (so we know not to call the callback).
But attaching hundreds of thousands of them is slow: it's a
single-linked list, which is iterated in several places.

But that's redundant: the request is now allocated off the cmd, so freeing the command
will free the request anyway.

Hacking in something to print progress to a file, here's the number of
requests processed every 10 seconds before and after:

Before:
	$ while sleep 10; do wc -l /tmp/bkpr-progress; done
	181529 /tmp/bkpr-progress
	195994 /tmp/bkpr-progress
	207083 /tmp/bkpr-progress
	226336 /tmp/bkpr-progress
	234319 /tmp/bkpr-progress
	241514 /tmp/bkpr-progress
	247421 /tmp/bkpr-progress
	255292 /tmp/bkpr-progress
	261367 /tmp/bkpr-progress
	269085 /tmp/bkpr-progress
	276953 /tmp/bkpr-progress
	282233 /tmp/bkpr-progress
	286193 /tmp/bkpr-progress
	290930 /tmp/bkpr-progress
	295276 /tmp/bkpr-progress
	301086 /tmp/bkpr-progress

After:
	169505 /tmp/bkpr-progress
	196010 /tmp/bkpr-progress
	219370 /tmp/bkpr-progress
	235671 /tmp/bkpr-progress
	244242 /tmp/bkpr-progress
	255362 /tmp/bkpr-progress
	265636 /tmp/bkpr-progress
	276966 /tmp/bkpr-progress
	284451 /tmp/bkpr-progress
	288836 /tmp/bkpr-progress
	296578 /tmp/bkpr-progress
	304571 /tmp/bkpr-progress

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

This significantly speeds up the query which bookkeeper often does:

		      "SELECT created_index"
		      " FROM channelmoves"
		      " WHERE payment_hash = X'%s'"
		      "   AND credit_msat = %"PRIu64
		      "   AND created_index <= %"PRIu64,

On large databases this scan is expensive, and a payment_hash index
cuts it down a great deal.  It does take longer to load the channelmoves
in the first place though (about 3x).

Before:
	$ while sleep 10; do wc -l /tmp/bkpr-progress; done
	169505 /tmp/bkpr-progress
	196010 /tmp/bkpr-progress
	219370 /tmp/bkpr-progress
	235671 /tmp/bkpr-progress
	244242 /tmp/bkpr-progress
	255362 /tmp/bkpr-progress
	265636 /tmp/bkpr-progress
	276966 /tmp/bkpr-progress
	284451 /tmp/bkpr-progress
	288836 /tmp/bkpr-progress
	296578 /tmp/bkpr-progress
	304571 /tmp/bkpr-progress

After:
	$ while sleep 10; do wc -l /tmp/bkpr-progress; done
	161421 /tmp/bkpr-progress
	238273 /tmp/bkpr-progress
	281185 /tmp/bkpr-progress
	305787 /tmp/bkpr-progress

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: plugins: the sql plugin now keeps an index on `channelmoves` by `payment_hash`.