vshard: recovery point manager vshard-router backend

Since: VShard 0.1.42

VShard now ships a backend for Tarantool's recovery point manager. It
lets a vshard cluster periodically create cluster-consistent recovery
points that can later be used for backup and point-in-time recovery of
the whole cluster. Note, that the whole cluster must be recovered to
that point, recovery of one or several replicasets is prohibited and
lead to doubled buckets.

The backend requires the sharding "router" role. Enable the backend on a
router instance by configuring the recovery point manager role with the
vshard-router backend, for example:

    roles: [roles.recovery-point-manager]
    roles_cfg:
      roles.recovery-point-manager:
        backend: vshard-router
        create:
          timeout: 120
          by:
            interval: 600

The cluster point is created with no bucket rebalancing in progress,
which makes it cluster-consistent.

The create.timeout option should be about 1.5 times bigger than the
storages' sharding.rebalancer_bucket_send_timeout, if recovery points
must be created during rebalancing. With too small timeout no recovery
point can be created for as long as rebalancing is running. Note that
rebalancer_bucket_send_timeout is infinite by default, so set it to a
finite value to make recovery points creatable during rebalancing.
Requested by @Serpentian in https://github.com/tarantool/vshard/commit/f7b998b6810ba0a0bf0f49427f2a6daff8622791.

No target file is named. Start by locating the documentation for the recovery point manager and vshard router role, using the supplied YAML configuration and timeout guidance as the source. Done means the vshard-router backend, whole-cluster recovery restriction, and rebalancing timeout requirements are documented clearly.