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.

For any change to a page or node, update all other cursors pointing
at the same page (or node). Cursors are now stored in a linked list
off their owning transaction. Cursors are all closed when the transaction
ends. Cursors in parent transactions are updated when their child
transaction commits.

Microoptimize IDL search.
Use RANGE_<FIRST/LAST> when IDL is known to be a range.

Check for stale DBs was in the wrong place.