feat(cgo): Implement graceful shutdown

[Fangop]

• The code has been formatted by gofmt
• The update of go.sum and go.mod has been synchronized by go mod tidy
• The graceful shutdown of dcurl cgo module will destroy dcurl while receiving SIGTERM and SIGINT signal as HORNET.
• When destroying dcurl, the logger will log the warning of destroying dcurl.

Graceful Shutdown

The graceful shutdown implementation requires 2 new go package to be imported.

• The package syscall is for the information of system call.
• The package os and its sub package os/signal is for notifying the signal of system
  The concept of the implementation is to start a goroutine when importing dcurl module.
  The goroutine will listen to signal of the system.
  When receiving SIGTERM and SIGINT, it'll log the warning and safely destroy dcurl.

Logger

The logger is based on the primitive log package in go.
Originally, I use the go.uber.org/zap(the logger iotaledger/hive.go/logger based on) for the logger.
Consider of the complexity of the synchronization of the loggers, I used primitive log package in go for its simplicity.

Thanks for the reviewing!