Connect is leaking connections when context is quickly canceled

[mherr-google]

Issue description

mysql.Connect is leaving open connections in cases when the context is canceled very quickly. Specifically, the error branch for watchCancel() in connector.go should call mc.cleanup() but doesn't:

	// Call startWatcher for context support (From Go 1.8)
	mc.startWatcher()
	if err := mc.watchCancel(ctx); err != nil {
		return nil, err
	}
	defer mc.finish()

We've reproduced this leak internally under heavy load, and that adding a call to mc.cleanup() fixes it.

Configuration

Driver version (or git SHA): c45f530

Go version: go version go1.13 linux/amd64

Server version: MySQL 5.7

Server OS: E.g. Debian 8.1 (Jessie), Windows 10

[mherr-google]

I'll send a pull request fixing this shortly.