memtx: add stubs to keep track of upgraded read view tuples

If a read view is created while space upgrade is in progress, tuples
fetched from the read view may be either upgraded or not. We need to
be able to differentiate those tuples so that we can use the appropriate
tuple format for them. To achieve that this commit adds the following
function stubs:

 - memtx_space_upgrade_track_tuple and memtx_space_upgrade_untrack_tuple
   will be used to maintain a set of all upgraded tuples.
 - memtx_read_view_tuple_needs_upgrade will do a lookup in the set of
   all upgraded tuples to check if a tuple needs upgrade.

The stubs will be implemented in the EE repository.

Note that we have to call memtx_space_upgrade_untrack_tuple from
memtx_engine_rollback_statement. The problem is that the space may be
deleted while a transaction is inprogress, in which case we must not
access space->upgrade in memtx_engine_rollback_statement. Fortunately,
we call memtx_tx_on_space_delete when a memtx space is altered to
rollback memtx transactions. So to handle this situation we set
txn_stmt->engine_savepoint to NULL from memtx_tx_history_remove_stmt
called from memtx_tx_on_space_delete. This makes the rollback function
return early.

Needed for tarantool/tarantool-ee#236

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