Resolve deadlock between pending CDR and batch CDR locks

r375757 attempted to resolve a race condition between multiple submissions of
CDRs while in batch mode from attempting to destroy the scheduled batch
submission by extending the batch CDR lock. Unfortunately, this causes a
deadlock between the pending CDR lock and the batch CDR lock. This patch
resolves the intent of r375757 by simply providing a new lock that protects
the scheduling of the batches. The original batch CDR lock is kept to protect
manipulation of the batch CDR settings, but has been placed such that it
is not held when the pending lock is held.

Thanks to Chase Venters for providing lock analysis on the issue.

(issue ASTERISK-21162)
Reported by: Chase Venters
........

Merged revisions 383839 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 383840 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@383841 65c4cc65-6c06-0410-ace0-fbb531ad65f3