tdb2: speed up transaction code by making page size a constant.

tdb2: speed up transaction code by making page size a constant.

Turning getpagesize() into a constant 4096 really helps gcc optimize, and
not just on 64 bit.

Here are the results of running "tools/speed --transaction --no-sync 2000000":

i386 with gcc 4.4.5:
Before:
	$ ./speed --transaction --no-sync 2000000
	Adding 2000000 records:  1195 ns (93594224 bytes)
	Finding 2000000 records:  719 ns (93594224 bytes)
	Missing 2000000 records:  429 ns (93594224 bytes)
	Traversing 2000000 records:  523 ns (93594224 bytes)
	Deleting 2000000 records:  901 ns (93594224 bytes)
	Re-adding 2000000 records:  1032 ns (93594224 bytes)
	Appending 2000000 records:  1711 ns (182801232 bytes)
	Churning 2000000 records:  3233 ns (182801232 bytes)

After:
	Adding 2000000 records:  868 ns (93594224 bytes)
	Finding 2000000 records:  569 ns (93594224 bytes)
	Missing 2000000 records:  369 ns (93594224 bytes)
	Traversing 2000000 records:  406 ns (93594224 bytes)
	Deleting 2000000 records:  674 ns (93594224 bytes)
	Re-adding 2000000 records:  730 ns (93594224 bytes)
	Appending 2000000 records:  1144 ns (182801232 bytes)
	Churning 2000000 records:  2085 ns (182801232 bytes)

Here's the effect (average of 10) on x8664 with gcc 4.1.2, with an earlier
tree and 1000000:
	Before:
		Adding 830.1ns
		Finding 428.3ns
		Missing 250.3ns
		Traversing 387.4ns
		Deleting 588.8ns
		Re-adding 737.2ns
		Appending 1141.4ns
		Churning 1792ns

	After:
		Adding 562.8ns
		Finding 362.1ns
		Missing 226.4ns
		Traversing 276.5ns
		Deleting 426.7ns
		Re-adding 489.8ns
		Appending 736.4ns
		Churning 1112.7ns