[RFC] Switch on tdb2 API by default?

[Rusty Russell]

Hi all!

   The TDB2 code (ie. --enable-tdb2) runs in TDB1 mode by default now
(there's a tdb_compat shim which inserts the TDB_VERSION1 flag unless
the env var TDB_COMPAT_USE_TDB2 is set).

   So I'd like to make --enable-tdb2 the default now.  That way we can
test the tdb2 API, and have a run-time flag to enable the tdb2 on-disk
format for the brave.

   This lets us get rid of some compat cruft; for example, tdb2 handles
multiple opens internally, so the various db_wrap codes wouldn't have
to.  We can also add a tdb2 alloc attribute to (finally!) make tdb2 use
talloc() for SAMBA.

I'm particularly interested in regressions if we turn this on.  For
example, tdb2 functions return the negative error code, not -1: I think
I got all the callers, but new ones may appear.

Thoughts?
Rusty.