Support for withdraw rate fee

[sjanel]

Sometimes the withdraw fee is not fixed but relative. Currently only fixed withdraw fee is managed by coincenter.

Example: XRP on Huobi:

{
  "addrDepositTag": true,
  "addrWithTag": true,
  "baseChain": "XRP",
  "baseChainProtocol": "XRPTOKEN",
  "chain": "solo",
  "depositStatus": "allowed",
  "displayName": "SOLO",
  "fullName": "XRP",
  "maxWithdrawAmt": "9911613.000000000000000000",
  "minDepositAmt": "5",
  "minWithdrawAmt": "5",
  "numOfConfirmations": 1,
  "numOfFastConfirmations": 1,
  "transactFeeRateWithdraw": "0.0002",
  "withdrawFeeType": "ratio",
  "withdrawPrecision": 7,
  "withdrawQuotaPerDay": "9911613.000000000000000000",
  "withdrawQuotaPerYear": null,
  "withdrawQuotaTotal": null,
  "withdrawStatus": "allowed"
}