tdb2: expand more slowly.

tdb2: expand more slowly.

We took the original expansion heuristic from TDB1, and they just
fixed theirs, so copy that.

Before:

After:
time ./growtdb-bench 250000 10 > /dev/null && ls -l /tmp/growtdb.tdb && time ./tdbtorture -s 0 && ls -l torture.tdb && ./speed --transaction 2000000
growtdb-bench.c: In function ‘main’:
growtdb-bench.c:74:8: warning: ignoring return value of ‘system’, declared with attribute warn_unused_result
growtdb-bench.c:108:9: warning: ignoring return value of ‘system’, declared with attribute warn_unused_result

real	1m0.243s
user	0m13.677s
sys	0m4.336s
-rw------- 1 rusty rusty 683302864 2011-04-27 21:03 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m24.074s
user	0m0.344s
sys	0m0.468s
-rw------- 1 rusty rusty 836040 2011-04-27 21:04 torture.tdb
Adding 2000000 records:  1015 ns (110551992 bytes)
Finding 2000000 records:  641 ns (110551992 bytes)
Missing 2000000 records:  445 ns (110551992 bytes)
Traversing 2000000 records:  439 ns (110551992 bytes)
Deleting 2000000 records:  807 ns (199517112 bytes)
Re-adding 2000000 records:  851 ns (199517112 bytes)
Appending 2000000 records:  1301 ns (376542552 bytes)
Churning 2000000 records:  2423 ns (553641304 bytes)