replication: set default replication_sync_timeout to 0

The only observable behaviour of non-zero replication_sync_timeout is
that it delays box.cfg{replication=...} return until either the node is
synced with others or the timeout passes.

If the timeout passes without reaching sync, box.cfg{} is exited and the
node enters "orphan" state, in which it can't write anything until
either a reconfiguration happens or replicaset is finally synced.

While the previous box.cfg{} call is running (probably waiting for
replication_sync_timeout), the user can't issue another box.cfg{} call.

So basically, while giving no guarantees that the node exits box.cfg{}
in fully synced state, the timeout makes reconfiguration harder: even if
the user knows that the sync won't be achieved, he will have to wait
until the full timeout passes in order to reconfigure replication.

Let's make the default value of replication_sync_timeout 0 instead of
300 seconds. The user still may set the timeout to whatever he likes.
Besides, we have recently introduced box.ctl.on_recovery_state triggers,
which have a "synced" event, and this is the new recommended way to wait
until the node is synced with others.

Part-of #5272

@TarantoolBot document
Title: Changed default value for `box.cfg.replication_sync_timeout

The default value for `replication_sync_timeout` configuration option
was changed from 300 seconds to 0.