lib/tdb2: Do not include config.h in (to-be) public library, use replace.

lib/tdb2: Do not include config.h in (to-be) public library, use replace.

Like tdb1, it's the caller's responsibility to set up various config
options (eg. by #include "config.h") before including the public
header.

We use HAVE_CCAN for including the (private) CCAN headers, otherwise
dummy macros are used.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>