Chunk server: move write id filed into ChunkAccessRequestOp super class of write rpcs in order to allow to create chunk access tokens if requested. Fix chunk access tokens generation by passing write id instead of chunk id as subject id.
Client lib: fix chunk close with write by setting chunk access before clearing write ids, and the first write id is used with chunk access subject.
Write started working with "security" / authentication enabled, though synchronous replication isn't tested yet.