Fix a memory leak in Lua stored procedures introduced
when cherry-picking changes from the old net-split
tree.

Make a palloc savepoint *before* we allocate txn
and port, to make sure txn and port memory is freed
when rolling back to the palloc savepoint.
Otherwise a long-running Lua procedure (e.g. background
fiber) can keep allocating palloc memory indefinitely.

Don't log an error when replication relay sees a partially read row.
This is not a corrupted log file, it's just a partial write by the wal
writer.

Implement blueprint update-append:
allow to push and pop fields at the end of the tuple,
without knowing the tuple size.

Use rope index to dynamically cut and paste fields
in a tuple.

Designate UINT32_MAX as a filed number for
tuple end. It's possible to SET or INSERT
fields at the end of the tuple, by specifying
field_no == UINT32_MAX. Delete
with field_no == UINT32_MAX deletes the last
field.

Add a test.

https://blueprints.launchpad.net/tarantool/+spec/update-field-relative-id
https://github.com/mailru/tarantool/wiki/UPDATE-request-implementation-%28ropes%29

*Incompatible change*

semantics of field indexes in UPDATE command conatining
more than one SET, DELETE or INSERT operation is
changed by this patch.

While before the patch, all field indexes were evaluated
before the update itself, after this patch, every
operation that changes the number of fields in a tuple
affects evaluation of field indexes of the subsequent
operation: i.e. indexes are evaluated against the "current"
tuple, as if all preceeding operations have been already
executed.

This patch changes the update implementation
to use a rope for field indexes. For details,
please refer to the comment near Update command,
and feature specification in Launchpad.

This is a prerequisite patch for UPDATE PUSH/POP
operations.

to the tuple:transform() function.

(https://bugs.launchpad.net/tarantool/+bug/1038784)

Conflicts:
	test/box_big/lua.result
	test/box_big/lua.test

Add rope implementation based on AVL trees.
Add unit tests.