tuple: rename update to xrow_update in tuple_update.c

First reason - update is a too general name. Tarantool
has SQL update, update in Lua, configuration update.
So name 'just update' can't be used.

'Xrow update' fits because it works directly with
MessagePack tuple internals and because the updates
are persisted in WAL in that format.

Second reason, without which the first one would not
matter - next patches are going to split
tuple_update.c into multiple module files. That will
make some structures and functions of tuple_update.c
be declared in header files, what makes them a public
API of xrow update.

Public API methods should be prefixed with their
subsystem name, and here it is 'xrow_update_'.

Part of #1261