tdb2: try to fit transactions in existing space before we expand.

Currently we use the worst-case-possible size for the recovery area.
Instead, prepare the recovery data, then see whether it's too large.

Note that this currently works out to make the database *larger* on
our speed benchmark, since we happen to need to enlarge the recovery
area at the wrong time now, rather than the old case where its already
hugely oversized.

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	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)

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	0m47.127s
user	0m17.125s
sys	0m2.456s
-rw------- 1 rusty rusty 366680288 2011-04-27 21:34 /tmp/growtdb.tdb
testing with 3 processes, 5000 loops, seed=0
OK

real	1m16.049s
user	0m0.300s
sys	0m0.492s
-rw------- 1 rusty rusty 244472 2011-04-27 21:35 torture.tdb
Adding 2000000 records:  894 ns (110551992 bytes)
Finding 2000000 records:  564 ns (110551992 bytes)
Missing 2000000 records:  398 ns (110551992 bytes)
Traversing 2000000 records:  399 ns (110551992 bytes)
Deleting 2000000 records:  711 ns (225633208 bytes)
Re-adding 2000000 records:  819 ns (225633208 bytes)
Appending 2000000 records:  1252 ns (248196544 bytes)
Churning 2000000 records:  2319 ns (424005056 bytes)

ccan/tdb2/private.h

@@ -222,7 +222,7 @@ static inline unsigned frec_ftable(const struct tdb_free_record *f)
 
 struct tdb_recovery_record {
 	uint64_t magic;
-	/* Length of record. */
+	/* Length of record (add this header to get total length). */
 	uint64_t max_len;
 	/* Length used. */
 	uint64_t len;