cdb_fetch()/cdb_next_item() return immutable data

cdb_fetch()/cdb_next_item() return immutable data

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Before now, the Berkeley DB back end was supplied the DB_DBT_MALLOC flag
and it became the caller's responsibility to handle any memory allocated
and returned by cdb_fetch() and cdb_next_item().  These heuristics are
not expected to be compatible with new back ends currently in planning.

cdb_fetch() and cdb_next_item() now return data that is to be considered
immutable (don't write to it or you'll segfault).  The caller must still
call cdb_free() to free the `struct cdbdata` itself, but the pointers
inside it point to buffers that the back end will reuse.

After a call to cdb_fetch() or cdb_next_item(), the buffers are
guaranteed to be readable until the next call to one of those functions
on the same table.