Remove race condition for queued issue notifications

project_issue_email_notify() is invoked during project_issue_node_insert() and project_issue_comment_insert(), in the middle of transactions to insert those. If the queue is drained at the same time, it is possible to beat the end of the insert. That sends out empty/incomplete issue notifications.

In my 40 seconds of searching, I didn't see hooks invoked after these operations. Queueing can be delayed until a shutdown function.