* fill in XDPoS_v2 variables and processQC/TC
* add timeout pool, refine engine variables
* refactor type functions
* solve a small pointer bug
* create general pool and its test, refine engine
* refine pool, add xdpos v2 config cert threshold
* refine config
* define vote, timeout, sync info, qc, tc, extra fields in types.go, add test in types_test.go
* add json tag in types.go, refine encoder decoder of extra fields
* refactor types.go utils.go
* re-write types, comments
* add Hash SigHash for types, and tests
* define Round type
* remove unnecessary logs