Extend docs with supported status returns. Add several helper functions
which allow to check the particular status.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
In previous implementation payload chunks were split into pieces with
512B length. This led to sending a large number of messages with a large
amount of payload.
Increase per-message payload chunk limit to 3MB.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
