Phase 1 - extract model api V2 (wip for Review) DO NOT MERGE

[Sing-Li]

• differentiate between code running on the client and on the server
• rename ModelUser to the more descriptive AccountsProvider
• only use (subclasses of) AccountsProvider on the server
• Remove DDP connection dependency from AccountsCommon
• remove setting of this.users from AccountsProvider (in discussion)
• Make user() abstract and implement separately on the client and server.
• Accept DDP connection on AccountsClient
• Move _initConnection to AccountsClient
• initialize this.users on the client.
• Client-side test code continue to use the minimongo API to access the users collection; full mutation support
• Pass AccountsProvider instance when creating AccountServer (figure out connection management - mongo vs other providers)
• Publish current user to users collection from server (TBD)
• Publish other users from server, design new API (TBD)
• Add this.accountsProvider on server
• implement backwards compatible method for caller wanting this.users when AccountsProvider is mongodb (in discussion)
• rename findSingle to findById
• replace/rename findCurrentUser and findSingleLoggedIn, perhaps with expanded findById (opting for rename vs combine, in consideration of non-mongo providers)
• rename insertHLoginToken to insertHashedLoginToken
• cleanup and/or remove findUsersInServices and getMongoUsersForReactiveWork
• pass all unit tests

[Sing-Li]

• added Meteor.dataProvider on server
• ensure all server code in unit tests access only Meteor.dataProvider and not Meteor.users
• added new unit tests for `Meteor.users' access compatibility when data provider is mongo
• use Meteor 1.3.2.4 dev bundle

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