Store index as space_id:index_id. Configuration refactoring.

Move write operations out of index into separate
sophia_space.cc file.

Fix memory corruption caused by accidentally copy-pasted line.

space_truncate() depends on box_delete()/box_insert(),
move to appropriate place.

 - Introduce new two-phase JOIN protocol.
 - Remove Sophia snapshots and switch to checkpoints.
 - Include latest WALs and registration of replica into JOIN stream.
 - Remove legacy tests for read-only replica.

See box_proccess_join() for protocol changes.

Fill IPROTO_KEY for DELETE and IPROTO_TUPLE for INSERT/REPLACE.

Splitted init_field_map into init_field_map and tuple_validate

* Remove pcall_wrapper()
* Use reflection to implement err:__index() and err:unpack()

Breaking changes:

 * pcall() returns 'struct error' cdata instead of string for box.error()
 * box.error.last() returns 'struct error' instead of Lua table

Refactor engine.cc to make a snapshot
based on existing, not future, WAL state.

Implement wal_checkpoint() which delivers WAL
vclock to tx thread and reopens the WAL.

* Remove recovery->apply_row and recovery_apply_row
* Remove struct recovery from sophia_engine
* Prepare to remove struct recovery from memtx_engine
* Prepare to remove struct recovery from applier

- Update link to configuration files
- Change number of queries and data set size
[skip ci]

Delete the previous checkpoint in sophia only if we have a new one.

Actually fix the bug and add a test case.

Actually fix the bug and add a test case.