replication: add META stage to JOIN

The new META stage is part of server's response to a join request.
It's marked by IPROTO_JOIN_META and IPROTO_JOIN_SNAPSHOT requests and goes
before the actual snapshot data.

Prerequisite #6034

@TarantoolBot document
Title: new protocol stage during JOIN

A new stage is added to the stream of JOIN rows coming from master.
The stage is marked with a bodyless row with type
IPROTO_JOIN_META = 71
Once all the rows from the stage are sent out, the JOIN continues as
before (as a stream of snapshot rows). The end of META stage is marked
with a row of type IPROTO_JOIN_SNAPSHOT = 72

The stage contains the rows that are necessary for instance
initialization (current Raft term, current state of synchronous
transaction queue), but do not belong to any system space.