Steps to reproduce: * Create a forum with a large number of posts on device A * Share the forum with device B * Accept the invitation on device B * Write private messages on device B while the forum posts are syncing Expected result: * Locally created messages appear in the private conversation immediately Actual result: * Locally created messages appear in the private conversation after a long delay What I believe is happening is that all the CryptoExecutor threads are busy validating forum posts. Creating a private message involves submitting a task to the CryptoExecutor, and those tasks get stuck behind the forum validation tasks. We could easily fix this for private message by moving creation off the CryptoExecutor, as creating a private message doesn't involve any long-running crypto operations, but the problem would still exist for forum posts, blog posts, etc. A proper fix would involve limiting the number of messages the validation manager queues for validation. At present there's no limit - all messages that require validation are queued on the CryptoExecutor and then passed to the DbExecutor. (This issue existed before the ValidationManager refactoring, but I didn't get round to reproducing it until now.) Related to #511.