Make low-level API for using katib flexibly

Discussde #66 This is the APIs I’m going to refactor and add.

API	input	process	output
CreateStudy	StudyConfig	Save Study conf to DB CreateStudyID	StudyID error
GetSuggestions	StudyID SuggestionAlgorithmName RequestNum	Create Trials from Suggesiton	[]TrialID error
RunTrials	StudyID []TrialID Worker	Request to run Trial to worker Set Trial status running	error
StopTrials	StudyID []TrialID IsComplete	Stop Trial worker Set Trial Status Complete	[]TrialID error
ShouldStopTrial	StudyID EarlyStopAlgorithm	Get ShuoulStop Trials	[]TrialID error
SetSuggestionParameter	StudyID SuggestionAlgorithmName AlgorithmParam	Set Parameters	error
SetEarlyStoppingParameter	StudyID EarlyStoppingAlgorithmName EarlyStoppingParam	Set Parameters	error
GetMetrics	[]TrialID	Get Metrics of Trials	[]Metrics error
SaveStudy	StudyID	Save StudyInfo to ModelDB	error
SaveModels	StudyID []TrialID []Metrics	Save Trial and Metrics Info to ModelDB	error

Typical usage is like below.

	studyId, _ := grpc.CreateStudy(studyConfig)
	grpc.SetSuggestionParameter(studyId, "random", suggestParam)
	grpc.SetEarlyStoppingParameter(studyId, "medianstopping", earlystopParam)
	grpc.SaveStudy(studyId)
	for IsStudyComleted() {
		trials, _ := grpc.GetSuggesitons(studyId, "random", 10)
		grpc.RunTrials(studyId, trials)
		for {
			metrics, workerState, _ := grpc.GetMetrics(studyId, trials)
			if AllWorkerCompleted(workerState) {
				grpc.CompleteTrial(studyId, trials, true)
				grpc.SaveModels(studyId, trials, metrics)
				break
			}
			shouldStops := grpc.ShouldStopTrial(studyId, trials)
			grpc.CompleteTrial(studyId, shouldStops, false)
			deleteShuldStopsFromTrialList(trials, shouldStops)
		}
	}

WDYT? @ddysher @gaocegege @libbyandhelen

[YujiOshima]

@libbyandhelen Oh, I’m sorry for my mistake. I will open PR to fix it.

[ddysher]

@YujiOshima thanks for putting this together! I’m on business travel last two days, will take a look ASAP 😃