reduce expected threads from 16 to 8 to improve general performance
Add CollectionFactory.getExpectedThreads

IP: fixed contract test
Allow override of DefaultPrefixManager in PrefixOWLOntologyFormat

This adds constructors and methods to inject dependencies. V4 has
better interfaces that makes this more useful.

Note this does not allow to determine the order of prefixes in the 
output. There is more code needing cleanup in the storers and renderers.
Removed redundant call
Replaced some references to DefaultPrefixManager with PrefixManager

Uses of DefaultPrefixManager that only used the PrefixManager interface
have been replaced.