Add assign_read_view() call

[ayurchen]

It is an optional call to inform provider about the read views (identified by GTID) used by application. Use case: snapshot isolation. Signature:

wsrep_status_t assign_read_view(wsrep_t* wsrep, wsrep_ws_handle_t* ws, const wsrep_gtid_t* rv);

Application passes read view ID to provider in rv. If rv == NULL provider uses the current internal value.