- 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.

Conflicts:
	src/box/memtx_engine.h
	src/box/sophia_engine.h

Conflicts:
	src/box/lua/schema.lua

This makes the engines less dependent on the global recovery
object (eventually this dependency will be removed altogether).

* introduce MemtxEngine::m_snap_dir
* begin using it during recovery
* in MemtxEngine::join() create a separate xdir object,
  to avoid races (the method is invoked from 'relay' thread)
* remove recovery_setup_panic() and pass panic_on_wal_error
  to recovery_new().
* remember the last checkpoint in memtx engine state and
  use it for replication join calls (rather than xdir_scan()
  to find the latest snapshot).
* remove recovery_has_data()
* move recovery_last_checkpoint() to memtx_engine.cc
* remove recovery::snap_dir
* use the vclock from memtx engine in join
* now a different error message is thrown
for the case when snapshot file is missing