Improve Build Expiry Performance

There are a number of flaws with the way build expiry works currently and can cause pain on larger instances. (http://jira.atlassian.com/browse/BAM-3949).

Some improvements that could occur include:
1. Reducing the scope of each session that is held (less stale data)
2. Ensuring that changes are persisted as soon as possible
3. Improve general performance to reduce the amount of time that the data is kept for.
4. Anyway we can reduce the amount of data stored in memory in general

... there are many others, these are the ones that would help allleviate situations as seen in the JIRA linked above.