tdb2: use direct access functions when creating recovery blob

tdb2: use direct access functions when creating recovery blob

We don't need to copy into a buffer to examine the old data: in the
common case, it's mmaped already.  It's made a bit trickier because
the tdb_access_read() function uses the current I/O methods, so we
need to restore that temporarily.

The difference was in the noise, however (the sync no-doubt
dominates).

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	0m45.021s
user	0m16.261s
sys	0m2.432s
-rw------- 1 rusty rusty 364469344 2011-04-27 22:55 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m10.144s
user	0m0.480s
sys	0m0.460s
-rw------- 1 rusty rusty 391992 2011-04-27 22:56 torture.tdb
Adding 2000000 records:  863 ns (110601144 bytes)
Finding 2000000 records:  565 ns (110601144 bytes)
Missing 2000000 records:  383 ns (110601144 bytes)
Traversing 2000000 records:  409 ns (110601144 bytes)
Deleting 2000000 records:  676 ns (225354680 bytes)
Re-adding 2000000 records:  784 ns (225354680 bytes)
Appending 2000000 records:  1191 ns (247890168 bytes)
Churning 2000000 records:  2166 ns (423133432 bytes)

After:
real	0m47.141s
user	0m16.073s
sys	0m2.460s
-rw------- 1 rusty rusty 364469344 2011-04-27 22:58 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m4.207s
user	0m0.416s
sys	0m0.504s
-rw------- 1 rusty rusty 313576 2011-04-27 22:59 torture.tdb
Adding 2000000 records:  874 ns (110601144 bytes)
Finding 2000000 records:  565 ns (110601144 bytes)
Missing 2000000 records:  393 ns (110601144 bytes)
Traversing 2000000 records:  404 ns (110601144 bytes)
Deleting 2000000 records:  684 ns (225354680 bytes)
Re-adding 2000000 records:  792 ns (225354680 bytes)
Appending 2000000 records:  1212 ns (247890168 bytes)
Churning 2000000 records:  2191 ns (423133432 bytes)