iproto: introduce FETCH_SNAPSHOT_CURSOR feature

This commit introduces FETCH_SNAPSHOT_CURSOR feature, which is available
only in EE. The feature is not returned in response to IPROTO_ID and is
not shown in box.iproto.protocol_features in Community Edition. Its id
is shown only in box.iproto.feature, which is a list of all available
features in the current version.

Needed for tarantool/tarantool-ee#741

NO_CHANGELOG=minor

@TarantoolBot document
Title: Document iproto feature FETCH_SNAPSHOT_CURSOR

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/#net-box-connect

FETCH_SNAPSHOT_CURSOR feature requires cursor FETCH_SNAPSHOT on the
server. Its ID is IPROTO_FEATURE_FETCH_SNAPSHOT_CURSOR. IPROTO version
is 8 or more, Enterprise Edition is also required.