tdb2: reduce transaction before writing to recovery area.

We don't need to write the whole page to the recovery area if it
hasn't all changed.  Simply skipping the start and end of the pages
which are similar saves us about 20% on growtdb-bench 250000, and 45%
on tdbtorture.  The more thorough examination of page differences
gives us a saving of 90% on growtdb-bench and 98% on tdbtorture!

And we do win a bit on timings for transaction commit:

Before:
$ time ./growtdb-bench 250000 10 > /dev/null && ls -l /tmp/growtdb.tdb && time ./tdbtorture -s 0 && ls -l torture.tdb && ./speed --transaction 2000000
real	1m4.844s
user	0m15.537s
sys	0m3.796s
-rw------- 1 rusty rusty 626693096 2011-04-27 21:28 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m17.021s
user	0m0.272s
sys	0m0.540s
-rw------- 1 rusty rusty 458800 2011-04-27 21:29 torture.tdb
Adding 2000000 records:  894 ns (110556088 bytes)
Finding 2000000 records:  569 ns (110556088 bytes)
Missing 2000000 records:  390 ns (110556088 bytes)
Traversing 2000000 records:  403 ns (110556088 bytes)
Deleting 2000000 records:  710 ns (244003768 bytes)
Re-adding 2000000 records:  825 ns (244003768 bytes)
Appending 2000000 records:  1262 ns (268404160 bytes)
Churning 2000000 records:  2311 ns (268404160 bytes)

After:
$ time ./growtdb-bench 250000 10 > /dev/null && ls -l /tmp/growtdb.tdb && time ./tdbtorture -s 0 && ls -l torture.tdb && ./speed --transaction 2000000
real	0m50.366s
user	0m17.109s
sys	0m2.468s
-rw------- 1 rusty rusty 564215952 2011-04-27 21:31 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m23.818s
user	0m0.304s
sys	0m0.508s
-rw------- 1 rusty rusty 669856 2011-04-27 21:32 torture.tdb
Adding 2000000 records:  887 ns (110556088 bytes)
Finding 2000000 records:  556 ns (110556088 bytes)
Missing 2000000 records:  385 ns (110556088 bytes)
Traversing 2000000 records:  401 ns (110556088 bytes)
Deleting 2000000 records:  710 ns (244003768 bytes)
Re-adding 2000000 records:  825 ns (244003768 bytes)
Appending 2000000 records:  1255 ns (268404160 bytes)
Churning 2000000 records:  2299 ns (268404160 bytes)