Parameter to configure database connection encryption

[isuldor]

--dbtls is a configurable parameter to set the go-sql-driver/mysql option for tls. [1]

1: https://github.com/go-sql-driver/mysql#tls

[JoeGruffins]

I don't need to supply a certificate?

[isuldor]

I'm glad you brought that up because I'm not sure, and frankly puzzled as to why go's mysql driver would require that with a valid certificate. On the one hand, a simple test script with http.Get("https://test.stakey.net/") will happily consume the letsencrypt cert. But running stakepoold with --dbtls=true --dbhost=test.stakey.net results in a [ERR] DB: Unable to establish connection to db: x509: certificate signed by unknown authority. This is while the standard mysql client connects with --ssl-verify-server-cert=on just fine. It's as if the mysql driver doesn't use the same root store as the http library.

Either way, adding a configurable pem file with mysql.RegisterTLSConfig for folks using self-signed certs makes sense. I also omitted it since I'm not exactly sure where and how it should be implemented.

[JoeGruffins]

This looks good to me, but I would like to enable custom certificates using the code you linked. If you don't feel comfortable doing it, I can in another pr.

[jholdstock]

This PR only adds support for stakepoold, but dcrstakepool also connects to the db. We should probably add support for both.

[dajohi]

@isuldor https://godoc.org/github.com/go-sql-driver/mysql#RegisterTLSConfig