Implement the `hbbft_perf` for JSON/RPC use, as requested in
helium/miner #873.

A large portion of this code originally lived entirely within
`miner_cli_hbbft.erl`, so rather than duplicate it, the essential
information gathering now happens in `miner_util.erl` (per Evan's
suggestion) and is then parsed and formatted in different ways
by the CLI (`miner_cli_hbbft.erl`) and the JSON/RPC handler
(`miner_jsronrpc_hbbft.erl`).

This is a cherry-pick commit to rescue a branch that has been
screwed up by a bad rebase/merge.