This patch introduces a new space option, group_id, which defines how
the space is replicated. If it is 0 (default), the space is replicated
throughout the entire cluster. If it is 1, the space is replica local,
i.e. all changes made to it are invisible to other replicas in the
cluster. Currently, no other value is permitted, but in future we will
use this option for setting up arbitrary replication groups in a
cluster. The option can only be set on space creation and cannot be
altered.

Since the concept of replication groups hasn't been established yet,
group_id isn't exposed to Lua. Instead, we use is_local flag, both in
box.schema.space.create arguments and in box.space output.

Technically, to support this feature, we introduce a new header key,
IPROTO_GROUP_ID, which is set to the space group id for all rows
corresponding to a space, both in xlog and in snap. Relay won't send
snapshot rows whose group_id is 1. As for xlog rows, they are
transformed to IPROTO_NOP so as to promote vclock on replicas without
any actual data modification.

The feature is currently supported for memtx spaces only, but it should
be easy to implement it for vinyl spaces as well.

Closes #3443

@TarantoolBot document
Title: Document new space option - is_local
If a space is created with is_local flag set in options, changes made to
the space will be persisted, but won't be replicated.