tdb2: Make TDB1 code use TDB2's open flags.

tdb2: Make TDB1 code use TDB2's open flags.

This means getting rid of TDB_VOLATILE (perhaps we should use an
attribute for that?), TDB_INCOMPATIBLE_HASH (use the
tdb_attribute_hash for that) and TDB_CLEAR_IF_FIRST (use the
tdb_attribute_openhook for that).

We also get rid of TDB_DISALLOW_NESTING: that's the default for TDB2.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit 22d0e0dc59fc9d7e0046fec6971ef478c2d604fd)