AbstractFieldLayoutManager.getFieldLayout(Issue) spends a lot of time loading issue type and project when it doesn't need to

This is a performance improvement for the issue navigator. Steps to reproduce:
1. Load a large dataset (e.g. the dataset we use for Grinder tests)
2. Hit the issue navigator with multiple concurrent requests while profiling. I used this script:

for i in `jot 2000`; do echo $i; done | parallel -j 5 'curl -s -X POST -u admin:admin "http://localhost:8081/secure/IssueNavigator!executeAdvanced.jspa?runQuery=true&_={}&pager/start=10"'

3. The AbstractFieldLayoutManager.getFieldLayout(Issue) method emerges as taking up a lot of time because it is loading the project and issue type objects needlessly. Strictly speaking, only their ids are necessary.

See attached screenshot.

cc: jwinters