Track allocations and invalid accesses to freed tmp pages.

Fix for failure when committing a txn that is a child of
some parent txn.

Don't compare cursors that aren't deep enough to match the
current cursor.

2-byte alignment isn't good enough for subpages

Sub-pages need room to grow...

NULL out env->pghead before writing it, so that its pages can't be used
to satisfy the write.

Only for DUPFIXED DBs. Still could do better.

Missed a MDB_RESERVE case

When putting a record, just make space for the data, don't copy it.
(Not compatible with MDB_DUPSORT, since the actual data is needed
to determine the insert location.)

Skip the original cursor that made the change

Create the data file using exactly the given pathname, and
the lock file using a suffix on the data file name.