box: make replication_sync_lag option dynamic

In #3427 replication_sync_lag should be taken into account during
replication reconfiguration. In order to configure replication properly
this parameter is made dynamic and can be changed on demand.

@locker: moved dynamic config check to box/cfg.test.lua

@TarantoolBot document
Title: recation_sync_lag option can be set dynamically
box.cfg.recation_sync_lag now can be set at any time.