People expressed an interest in having access to the exact length of calls to a finer degree than seconds. See the CHANGES and UPGRADE.txt for usage also updated the sample configs to note the change.

Patch by snuffy.

(closes issue #16559)
Reported by: cianmaher
Tested by: cianmaher, snuffy

Review: https://reviewboard.asterisk.org/r/461/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@269153 65c4cc65-6c06-0410-ace0-fbb531ad65f3