When collapsing root, must adjust entire cursor stack

Was using the pre-touch dst page pointer instead of the touched page

When moving a node from one page to another, make sure other cursors'
parent index gets adjusted too.

When moving a node from the right neighbor, a different
adjustment is needed.

Don't adjust other cursors when we added a node on the right.

ITS#8258, ITS#7829 fixes checked parent index to see if we were moving
from a left neighbor. Should have just checked to see if current index
was 0, meaning we added on the left. (Parent index may not tell us
anything meaningful after a nested rebalance.)

Parent mc_ki wasn't adjusted if new_indx was > split point

The parent's mc_ki has changed. We need to fix that up as well
in other cursors.

No effect on current code.

...early enough that txn state is left unchanged.

Move the check to the end of txn startup.  Catches env
breakage which happens while the new txn waits for a lock.

* Add MDB_TXN_FINISHED, MDB_TXN_HAS_CHILD, MDB_TXN_BLOCKED.
* Clear mt_numdbs in writers, for TXN_DBI_EXIST() to catch.
  We already do in readers.

Side effects:
* Clean txn up a bit even before freeing it.
* Tweak DEBUG output at txn end. Add DEBUG after commit(writer).

MDB_DBI_CHANGED(,MAIN_DBI) is never true.

mt_env is always set.
Commit(mt_child) resets mt_child, so parent need not.