box: add replication_sync_timeout configuration option

In the scope of #3427 we need timeout in case if an instance waits for
synchronization for too long, or even forever. Default value is 300.

Closes #3674

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

@TarantoolBot document
Title: Introduce new configuration option replication_sync_timeout
After initial bootstrap or after replication configuration changes we
need to sync up with replication quorum. Sometimes sync can take too
long or replication_sync_lag can be smaller than network latency we
replica will stuck in sync loop that can't be cancelled.To avoid this
situations replication_sync_timeout can be used. When time set in
replication_sync_timeout is passed replica enters orphan state.
Can be set dynamically. Default value is 300 seconds.