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This change is lays groundwork for making attachment upload operations
persistent.

Make GenericChangeProcessor responsible for writing attachments to the
AttachmentStore and calling UploadAttachments.  In a future CL, datatype
code will be responsible for writing attachments to the store.

Queue up attachments for upload inside of AttachmentService.  In a
future CL we'll add rate limiting, retry, and back-off logic.

Expose AttachmentService's AttachmentStore via GetStore method.

BUG=

Review URL: https://codereview.chromium.org/512413003

Cr-Commit-Position: refs/heads/master@{#293536}