[Sync] ModelTypeStoreImpl implementation

This is implementation of ModelTypeStore interface.
Calls are forwarded to ModelTypeStoreBackend with ids formatted into
record keys. Typical pattern for cross-thread communication is that
API function creates scoped_ptr variables that will hold future result.
These variables are passed as pointers to task that invokes backend. The
ownership of these variables is passed to reply object which will be
posted when backend function completes.

This change lacks tests involving persistence across store initialization
and tests with faulty env. These will be added in separate change.

R=stanisc@chromium.org,skym@chromium.org
BUG=517663

Review URL: https://codereview.chromium.org/1460333002

Cr-Commit-Position: refs/heads/master@{#361528}

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