As a note to the peanut gallery, this PR is in line with what @buck54321 and I discussed on how best to implement this without "poisoning" dcrdata.

• It is not always desirable for dcrdata to be contacting external sources (besides the chain server)
• dcrdata's no-exchange-monitor and the individual exchange disabling option makes dcrdata's use of this behaviour is entirely optional (since it must be enabled with --exchange-montior).
• Containing the functionality to its own package opens the door to implementing a separate rate aggregator service.

What is not done in this PR (and that is OK), is developing a stand-alone rate server (e.g. dcrrates), which would:

1. Aggregate exchange data
2. Serve the data via HTTP or (g)RPC
3. Run on an entirely separate machine from dcrdata.
4. Supply data to 1 or more dcrdata instances.

I feel these features of a "dcrrates" process are important since dcrdata is a backend service that is often run on many instances behind a load balancer. It is not necessary for each instance to be hitting up the exchanges. It is also a security concern having the backends communicating with other sites, not only because dcrdata's outgoing http requests to the exchange APIs are a direct risk to the dcrdata process, but it becomes challenging to conceal identity of the backends (would need NAT, a proxy, etc.).

This LGTM. Will leave open and solicit opinions again today.

Does it overlap with https://github.com/raedahgroup/dcrextdata ?

@xaur, yes and no.

dcrextdata has postgresql storage for a different aim. dcrextdata is a PoC with just poloniex data.

dcrdata's exchange package (this PR) has a more formal and generalized architecture with type Exchange interface, which is currently implemented for 7 different exchanges.

They are similar in ways, but a new package for dcrdata was the best choice IMO.

I was getting ready to merge this, but I see there are conflicts in home.tmpl from the recent design work you and gozart were doing. There's also a conflict explorerroutes.go, but it's trivial.

Draft merge commit message:

This adds exchange rate monitoring and a new exchanges package.

The exchanges package contains ExchangeBot, which can be used to
monitor DCR-BTC exchanges and BTC-fiat indices. ExchangeBot must
be explicitly enabled, and individual exchanges can be disabled. Both
huobi and dragonex are disabled by default.
Currently it has 5 DCR-BTC exchanges and 2 BTC-fiat index sources.

A new API endpoint (/api/exchange) is added for retrieving the Decred
exchange data in one of almost 200 base currencies (indices). Results
are JSON-formatted and support indenting.

Initially, the price is simply displayed in a chosen default base on the
homepage.

Request time is logged before request is sent to prevent multiple
requests while the connection times out.

Ah, never mind. My local branch didn't have your force-push. All good now.