This shows the benefits of batching when being slammed by autoclean:
Before:
```
plugin-autoclean: last 1000 deletes took 6699022 nsec each
plugin-autoclean: last 1000 deletes took 6734025 nsec each
plugin-autoclean: last 1000 deletes took 6681189 nsec each
plugin-autoclean: last 1000 deletes took 6597148 nsec each
plugin-autoclean: last 1000 deletes took 6637085 nsec each
plugin-autoclean: last 1000 deletes took 6801425 nsec each
plugin-autoclean: last 1000 deletes took 6788572 nsec each
plugin-autoclean: last 1000 deletes took 6603641 nsec each
plugin-autoclean: last 1000 deletes took 6642947 nsec each
plugin-autoclean: last 1000 deletes took 6590495 nsec each
plugin-autoclean: last 1000 deletes took 6695076 nsec each
plugin-autoclean: last 1000 deletes took 6477981 nsec each
```
After:
```
plugin-autoclean: last 1000 deletes took 342764 nsec each
plugin-autoclean: last 1000 deletes took 375031 nsec each
plugin-autoclean: last 1000 deletes took 357564 nsec each
plugin-autoclean: last 1000 deletes took 381581 nsec each
plugin-autoclean: last 1000 deletes took 337989 nsec each
plugin-autoclean: last 1000 deletes took 329391 nsec each
plugin-autoclean: last 1000 deletes took 328322 nsec each
plugin-autoclean: last 1000 deletes took 372810 nsec each
plugin-autoclean: last 1000 deletes took 351228 nsec each
plugin-autoclean: last 1000 deletes took 413885 nsec each
plugin-autoclean: last 1000 deletes took 348317 nsec each
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>