Get rid of all of the redundant MDB_*page structs and other cruft.

Also fix xcursor_init1 md_dirty flag
Also another mdb_cmp refactoring fix

Don't allow new pages for free list to come from the free list.
Otherwise a nasty data self-reference occurs that is too much
trouble to unwind.

Can re-use free pages 1 txn earlier
Must update all dbs tables on txn commit

Also in cursor_set, check the current page (if already set from
before) before starting over again from the root.

For better space utilization

If O_DSYNC is not defined, that may be due to poor compiler options.
We could fall back to the less efficient O_SYNC, but it seems better
to let the user learn of the problem and give better compiler options.