config and log rearchitect

[Reasno]

This PR intends to build easy-to-use implementations on top of easy-to-extend interfaces, and bring the value of both to the package core.

The interface is easy to extend if there are very few methods. However, an interface with very few methods can be cumbersome to use.

Easy to extend:

type ConfigUnmarshaler interface {
	Unmarshal(path string, o interface{}) error
}

type Logger interface {
    Log(keyvals ...interface{}) error
}

Easy to use:

type ConfigAccessor interface {
	ConfigUnmarshaler

	String(string) string
	Int(string) int
	Strings(string) []string
	Bool(string) bool
	Get(string) interface{}
	Float64(string) float64
}

type LevelLogger interface {
	Logger
	Debug(args ...interface{})
	Info(args ...interface{})
	Warn(args ...interface{})
	Err(args ...interface{})
	Debugf(template string, args ...interface{})
	Infof(template string, args ...interface{})
	Warnf(template string, args ...interface{})
	Errf(template string, args ...interface{})
	Debugw(msg string, fields ...interface{})
	Infow(msg string, fields ...interface{})
	Warnw(msg string, fields ...interface{})
	Errw(msg string, fields ...interface{})
}

This PR allows user to upgrade the easy-to-extend interfaces to easy-to-use interfaces with one wrapped method call:

func WithAccessor(unmarshaler contract.ConfigUnmarshaler) contract.ConfigAccessor
func WithLevel(logger log.Logger) LevelLogger

So implementors only need to satisfy the minimum interfaces, but consumers are free to use the richer API provided by upgraded interfaces.

[codecov]

Codecov Report

Merging #177 (debb5ee) into master (4cccba0) will increase coverage by 0.02%.
The diff coverage is 83.07%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
+ Coverage   80.50%   80.53%   +0.02%     
==========================================
  Files          91       91              
  Lines        3273     3308      +35     
==========================================
+ Hits         2635     2664      +29     
- Misses        463      469       +6     
  Partials      175      175              

Impacted Files	Coverage Δ
default_config.go	78.94% <ø> (ø)
leader/dependency.go	83.72% <ø> (ø)
otes/dependency.go	80.32% <ø> (ø)
otetcd/dependency.go	97.22% <ø> (ø)
otgorm/dependency.go	91.89% <ø> (ø)
otkafka/dependency.go	86.36% <ø> (ø)
otmongo/dependency.go	85.71% <ø> (ø)
otredis/dependency.go	84.41% <ø> (ø)
ots3/dependency.go	67.39% <ø> (ø)
logging/log.go	40.50% <28.57%> (+1.06%)	⬆️
... and 8 more

---

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[lingwei0604]

LGTM