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box: introduce box.ctl.promote

Replicaset master promotion is a procedure of atomic making one
slave be a new master, and an old master be a slave in a fullmesh
master-slave replicaset.

The promotion follows the protocol described in details in the
corresponding RFC. Shortly, the protocol collects a quorum of
instances who approves the promotion, syncs data with the old
master and demotes it.

The protocol is intended to work with a single master cluster and
with at least 50% + 1 quorum mandatory including an old master.
It is tolerant to messages reordering from different sources, to
errors like multiple masters, timeouts, restarts of any promotion
participant. Also the promote protocol supports promotion in a
completely read-only cluster. It is useful, for example, when
after one of rare cases of a promotion fail the cluster is left
in a read-only state with no masters. Then the promotion can just
be called again to fix it. Such read-only promotion has only one
restriction - all of the instances have to be safe and sound.

Once a promotion is executed, it makes box.cfg.read_only
attribute be immutable. It is because actually the promotion
protocol persists this attribute as a part of one of messages and
sends it to other instances. So a user can not both use the
promotion and manually change box.cfg.read_only.

The promotion has several API methods:

* box.ctl.promote({timeout = ..., quorum = ...}).
  This function is meant to be called on a slave to demote the
  old master if exists and promote the current instance.

* box.ctl.promote_info().
  This function shows info about the latest promotion (finished
  or running now - does not matter, just the latest).

* box.ctl.promote_reset().
  This function clears the promotion history so a user would be
  able to re-assign master/slave roles in a cluster manually.

Closes #3055

@TarantoolBot document
Title: Document box.ctl.promote()
Subj. For details of the patch see the commit message. For
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