Rpc mempool: Support more fields in getmempoolinfo result.

[moodyjon]

Fixes #60

Field "loaded" only makes sense if the Mempool is being saved/loaded on restart. I don't see that happening.
Field "maxmempool" would need more configuration infrastructure, and could be supported if that were added.

The "usage" (memory) and "unbroadcastcount" were my attempt to mimic what's done in C++ bitcoin (https://github.com/bitcoin/bitcoin).

Manual test, showing rollover back to near 0 size, apparently on new block acceptance.

Mac-mini lbcd % ./lbcctl --rpcuser=xxx --rpcpass=yyy getmempoolinfo
{
  "size": 267,
  "bytes": 256195,
  "usage": 347789,
  "total_fee": 0.21047744,
  "mempoolminfee": 0.00001,
  "minrelaytxfee": 0.00001,
  "unbroadcastcount": 0
}
Mac-mini lbcd % ./lbcctl --rpcuser=xxx --rpcpass=yyy getmempoolinfo
{
  "size": 3,
  "bytes": 2260,
  "usage": 3223,
  "total_fee": 0.001164,
  "mempoolminfee": 0.00001,
  "minrelaytxfee": 0.00001,
  "unbroadcastcount": 0
}

Investigating automated tests next...

[coveralls]

Pull Request Test Coverage Report for Build 2691830427

• 78 of 129 (60.47%) changed or added relevant lines in 4 files are covered.
• 1064 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.08%) to 51.904%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcserver.go	0	1	0.0%
server.go	0	5	0.0%
mempool/memusage.go	43	59	72.88%
mempool/mempool.go	35	64	54.69%

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	76.47%
btcec/signature.go	3	92.82%
btcjson/walletsvrcmds.go	14	95.92%
claimtrie/claimtrie.go	21	75.21%
rpcclient/wallet.go	362	0%
rpcserver.go	662	0.3%

Totals
Change from base Build 2653196456:	0.08%
Covered Lines:	22543
Relevant Lines:	43432

---

💛 - Coveralls

[roylee17]

Nitpick:

I think it looks better without this though. Only four call sites with additional params.
Or, at least, rename the util function without leading _

[roylee17]

Looks good to me. And encouraged to upstream this to btcd as it's a general improvement.