xrow: rename xrow_encode_vclock

This commit renames xrow_encode_vlock to xrow_encode_vclock_ignore0
since the next commit will introduce encoding vclock without ignoring
0th component, which is needed during sending the response to fetch
snapshot request.

This commit also removes internal field inside the replication_request
structure, as the following commit will use 'vclock' for
encoding/decoding vclock without ignoring component.

Needed for tarantool/tarantool-ee#741

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring