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

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

`struct si` has longer lifecycle than `PhiaIndex`.

- Replace sfscheme->field_count with sfscheme->keys_count + 1.
- Keep only key parts in sfscheme->fields, remove value

- Remove position_ref, position_key, key from sffield
- Remove keys from sfscheme
- Remove sf_schemeadd and sc_schemevalidate
- Use uint32_t instead of int

Prepare to switch to Msgpack.
MsgPack is variable-length encoding, even for uint64 number.