Fix message-level modifications inferred from thread update API.
Fix message-level modifications inferred from thread update API.

Previously, updating a thread's labels would cause all those labels to be
applied to each message, which is wrong. For example, archiving a thread you've
replied to would cause the API to try to apply the 'sent' label to each
message.

Fixes T2325.

jobscore/sync-engine,gale320/sync-engine,closeio/nylas,ErinCall/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,closeio/nylas,gale320/sync-engine,ErinCall/sync-engine,closeio/nylas,ErinCall/sync-engine,Eagles2F/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,gale320/sync-engine,gale320/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,closeio/nylas,nylas/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,nylas/sync-engine