client: added the ability to validate the configuration

[dwagin]

For recreating the client, need the ability to check the configuration separately.

in continuation #195.

[twmb]

I think that since NewClient does more initialization and validation than this, the Validate function here isn't comprehensive enough to all the errors that could be returned. Also, I think the name needs to be a bit more specific, either ValidateOpts or CheckNewClient or ValidateNewClient or something.

I'd split the two goroutines started in NewClient into something like func (*Client) start(), add func (*Client) init(...Opt) error, and then have NewClient be init(opt...); errcheck; start() and ValidateOpts to be just return init().

Although this runs the risk someday of accidentally adding a goroutine in the middle of NewClient, so then add a doc line warning maintainers in a year not to add goroutines.

Also, since 1.7 has just released, this will probably wait a bit until a few more features for 1.8 stack up, perhaps a month or three in the worst case.

[twmb]

I've gone ahead and applied my suggestions above and committed this in 7ffb6e1 (co-author commit). Closing this PR now as duplicate -- this will be released in 1.8