Huge witness update

Market
* _weighted_average, btc_usd_ticker, steem_btc_ticker and steem_usd_implied added
Witness
* get_votes_sum added to witnesses object
Graphenerpc
* fix handling of WebSocketConnectionClosedException
CLI
* witnessfeed: uses the new market ticker to calculate the current steem_usd price as base. When using support-peg the sbd_usd price is calculated from the internal ticker and used for quote.
* Witness: shows the rank and active rank (when it is below 250). virtual_time_to_block_num is calculated from num_scheduled_witnesses, VIRTUAL_SCHEDULE_LAP_LENGTH2 and vote_sum. This is then used to estimate the next block producing time.
* witnesses uses a proxy now, when set.