DDL commit for replicas results in dead lock

When applying DDLCommit replica blocks raft loop, until lsn is updated, which doesn't get updated, until replica advances raft state machine.

Also not strictly the same, but related is a problem when applying the raft snapshot. Replicas must also synchronize with master before applying raft snapshot. More details in https://docs.google.com/document/d/1MEpGnpKKj6WezLKytvvonZzbpy1tlWtAK5ccxaeAOrE