Some changes can be missed when a device use twice the same synckey

Reproduction steps

None, this can be hard to reproduce but you can try these steps:

• synchronize usera emails with a device
• cut the device network
• send to usera a bunch of 1000 emails
• reconnect the device network
• as soon as it starts to make a request (see logs), cut the network again
• reconnect the device network
• let the device synchronize a bunch of new emails (~100 emails)
• make some changes with another MUA (tb, roundcube, ..) like deletions or read status updates

Current behavior

1. NOK deletions are not took in account by the device, even by forcing new syncs

Expected behavior

1. OK deletions are took in account by the device, may be that you will need to force new syncs

Note:
I've written tests for this jira, I've also tried a fix but we should evaluate another way to make that. See thilaire/OP-231