ldb: perf test for pack format

ldb: perf test for pack format

Performance test for pack function and unpack function run with flag
LDB_UNPACK_DATA_FLAG_NO_DATA_ALLOC. Run this test before and after
forthcoming pack format changes to test throughput improvement. On my
machine, the unpack improvement is around 50%.
The test doesn't really belong in LDB torture but it's the only place
where all the required functions are available.

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>