Decoupling DB interface from katib manager

[achalshant]

Goal: To decouple the DB interface from katib manager in order to make the backend store pluggable. Please refer to the proposal doc for more details - https://docs.google.com/document/d/1xAi4V9SCAVZvrwiVc6a3VpK7BXPx-liQbr6YPdqBThs/edit#heading=h.1hri22e2jta

Major changes:

pkg/db/interface.go was moved to run as a service in cmd/dbif/mysql/main.go
cmd/manager/main.go is now a thin client that connects to the DBIF service using GRPC and operates on the DB.
Some services and messages from api.proto have moved to dbif.proto.

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This change is 

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[YujiOshima]

please move pkg/db/v1alpha2/db_init.go and pkg/db/v1alpha2/interface_test.go to cmd/dbif/mysql/ since they are only for mysql.

[achalshant]

@hougangliu @YujiOshima

[hougangliu]

@achalshant thanks for your PR. I am afraid that we cannot include this feature in 0.6 release.
@johnugeorge @richardsliu @gaocegege any comment?

[richardsliu]

Agree, we can try to take it up in the next release in June or July.

[hougangliu]

@achalshant will you pick up this PR now?

[achalshant]

Okay @hougangliu .

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[k8s-ci-robot]

@achalshant: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
kubeflow-katib-presubmit	aa9eb3f	link	/test kubeflow-katib-presubmit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[johnugeorge]

DB interface is introduced in #685

[johnugeorge]

Closing this issue as the backend is pluggable