tdb2: limit coalescing based on how successful we are.

tdb2: limit coalescing based on how successful we are.

Instead of walking the entire free list, walk 8 entries, or more if we
are successful: the reward is scaled by the size coalesced.

We also move previously-examined records to the end of the list.

This reduces file size with very little speed penalty.

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	1m17.022s
user	0m27.206s
sys	0m3.920s
-rw------- 1 rusty rusty 570130576 2011-04-27 21:17 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m27.355s
user	0m0.296s
sys	0m0.516s
-rw------- 1 rusty rusty 617352 2011-04-27 21:18 torture.tdb
Adding 2000000 records:  890 ns (110556088 bytes)
Finding 2000000 records:  565 ns (110556088 bytes)
Missing 2000000 records:  390 ns (110556088 bytes)
Traversing 2000000 records:  410 ns (110556088 bytes)
Deleting 2000000 records:  8623 ns (244003768 bytes)
Re-adding 2000000 records:  7089 ns (244003768 bytes)
Appending 2000000 records:  33708 ns (244003768 bytes)
Churning 2000000 records:  2029 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	1m7.096s
user	0m15.637s
sys	0m3.812s
-rw------- 1 rusty rusty 561270928 2011-04-27 21:22 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m13.850s
user	0m0.268s
sys	0m0.492s
-rw------- 1 rusty rusty 429768 2011-04-27 21:23 torture.tdb
Adding 2000000 records:  892 ns (110556088 bytes)
Finding 2000000 records:  570 ns (110556088 bytes)
Missing 2000000 records:  390 ns (110556088 bytes)
Traversing 2000000 records:  407 ns (110556088 bytes)
Deleting 2000000 records:  706 ns (244003768 bytes)
Re-adding 2000000 records:  822 ns (244003768 bytes)
Appending 2000000 records:  1262 ns (268404160 bytes)
Churning 2000000 records:  2320 ns (268404160 bytes)