Different indexes cannot be commited at same time

In new indexing mechanism all 4 different independent indexes cannot be commited (flushed) simultaneously.
It could cause performance degradation on big amount of updates.

Explanation of this behaviour is in com.atlassian.jira.index.DefaultIndexEngine.FlushPolicy. Method perform is synchronised on static enum instance which shared across all index writers. There should be another lock used index type (issues, comments etc) based.