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

[Sing-Li]

• [x] differentiate between code running on the client and on the server
• [x] rename ModelUser to the more descriptive AccountsProvider
• [x] only use (subclasses of) AccountsProvider on the server
• [x] Remove DDP connection dependency from AccountsCommon
• [ ] remove setting of this.users from AccountsProvider (in discussion)
• [x] Make user() abstract and implement separately on the client and server.
• [x] Accept DDP connection on AccountsClient
• [x] Move _initConnection to AccountsClient
• [x] initialize this.users on the client.
• [x] 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)
• [x] Add this.accountsProvider on server
• [ ] implement backwards compatible method for caller wanting this.users when AccountsProvider is mongodb (in discussion)
• [x] rename findSingle to findById
• [ ] replace/rename findCurrentUser and findSingleLoggedIn, perhaps with expanded findById (opting for rename vs combine, in consideration of non-mongo providers)
• [x] rename insertHLoginToken to insertHashedLoginToken
• [ ] cleanup and/or remove findUsersInServices and getMongoUsersForReactiveWork
• [x] pass all unit tests

[Sing-Li]

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

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