Db tidy

This makes a number of minor changes to reduce the amount of redundant data recorded in the suite db as requested by @matthewrmshin and adds in some missing information:

 * redundant messages removed from task events table e.g. we now have: `|submitted||` instead of `|submitted|task submitted|`
 * in the task events table, the host is only recorded in the `submitted` event - this also fixes a bug whereby the owner@host value wasn't recorded right in the submitted event
 * task signal messages are now recorded in the task events tables as a "signaled" event i.e. any message starting "Task job script received signal" is now recorded
 * an entry is recorded in the task events table when a task enters the retry state