Websocket improvements and request queuing

Additions & improvements

• Replaced websocket package logs with channel errors
• Improved websocket package structure
• Added support for websocket header and origin
• Added support for custom websocket timeout configuration
• Created dedicated dispatchers for managing ocpp-j state and transmission
• Added support for multiple pending requests

Bugfixes

• Fix panics and logs in 1.6 examples
• Use SANs in x509 certificates, due to breaking changes in Go 1.15