This is required to do the following:

tarantoolctl user@host
tarantoolctl guest@host
tarantoolctl guest:@host

Currently all of those will lead to error from net.box. Either because
password is not specified, or because guest user can't login with empty
password.

This change effectively sets guest password to an empty string, to make
it less of a special case to authentication.

And since now guest can be authenticated with an empty password, net.box
will replace nil passwords with empty strings, when user name is
provided.

We still support the case when username=nil and password=nil. In this
case net.box doesn't send authentication packet at all. Guest login is
just assumed.

The bootstrap snapshot will now have guest user with an empty password,
instead of no password.

Fixes #1545

This happens because console:eval swallows errors
and doesn't print them to stdout.

Also, console.connect() doesn't always call on_client_disconnect()
in case of errors, so the code that was meant to terminate local
session on errors, didn't do this in some cases and caused local
console to remain open.

Fixes #1586

box_tuple_extract_key, tuple_extract_key, tuple_extract_key_raw
- functions for extracting key of any index from tuple or
raw msgpuck.

Fixed Lua code block formatting in box_index.rst

Prepare to change vinyl API to use (key, part_count) and (struct tuple).

No semantic change, just allow tx=NULL in vinyl_get() and remove
duplicated code.

Prepare to change vinyl API to use (key, part_count) and (struct tuple).

* Remove REF_FE/REF_BE magic code
* Ref count indexes in phia_cursor()
* Don't ref/unref count ALL indexes in phia_begin()/phia_tx_commit()
* Extract SI_SHUTDOWN/SI_DROP cases from si_execute()
* Fix handling of NO-OP task in planner
* Check sc_version after coeio call in phia_iterator_next()
* Add assertions and TRASH() to planner, index and tuples

Attempt to fix buggy phia_index refcouting. Now phia works without segfaults
more than 10 minutes on my host instead of 20-30 sec. DDL still may crash
in some situations.

Fix race condition in phia_service_do()/phia_get()

Regression introduced by 335391d0

Get rid of one-letter `i` variables.

- Move phia_tuple_from_data() and phia_tuple_from_key() to phia.c
- Make tuple_upsert_execute() to be C-friendly
- Move phia_upsert_cb() and related stuff to phia.c

Prepare to switch to MessagePack.

- Fix code style
- Remove useless tnt_raise() from tuple_compare_field()
- Use mp_decode_strbinl() for bin/str comparison