Support remote sync policy

[bhou-crto]

This feature allows developer to control the frequency of syncing with the remote repository

As a first step, developer needs to modify the configuration file manually to setup up the sync policy of the extra remote registry.
Ex.

{
            "name": "test",
            "remote_base_url": "https://url-to/test-registry",
            "repository_dir": "/Users/b.hou/.cdt/test",
            "sync_policy": "weekly"
}

By default, the sync_policy is set to 'always'. The possible values are:

• always
• never
• hourly
• daily
• weekly
• monthly

Updater will take the sync_policy into consideration before executing the update.

Note

the built-in update command cl update --package ignores the sync_policy, which "always" performs the update

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Caveats and Follow-up

Follow up improves

• check the sync_policy in the "preRun" phase, to improve the performance when there are too many remote registry.
• extract the constants of the sync_policy and use it in different modules
• provide command interface in the built-in remote command to change the sync policy

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This PR closes #106