HST Diagnostics - Add unit threshold time millisec configuration

In HST Diagnostics [1], I guess we can add @hst:diagnosticsunitthresholdmillisec (long) for unit threshold.
So, when the property is set to a non-negative value (>=0), it should not print out any HDC task taken less than the threshold value.

[1] http://www.onehippo.org/library/concepts/request-handling/hst-page-diagnostics.html