tdb: update README a bit

tdb: update README a bit

While studying tdb, I've noticed a couple of mismatches between readme
and actual code:

- tdb_open_ex changed it's log_fn argument to log_ctx
- there is now no tdb_update(), which it seems was transformed into
  non-exported tdb_update_hash()

There were other mismatches, but I don't remember them now, sorry.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>