Expensive default variable creation

[rosstimothy]

The RSA keys generated for the defaultServer global variable have a dramatically negative impact on any consumers of the server package.

go-mysql/server/server_conf.go

Line 11 in 639eccd

var defaultServer = NewDefaultServer()

This can be measured using GODEBUG='inittrace=1' on any programs that imports github.com/go-mysql-org/go-mysql/server.

From a M2:

212 ms github.com/go-mysql-org/go-mysql/server

From a t3.xlarge:

537 ms github.com/go-mysql-org/go-mysql/server

[lance6716]

I think we can use sync.Once to lazily create the defaultServer. Welcome to contribute

[dveeden]

@rosstimothy could you check #1021 ?