core: Don't create snapshots with locks.

Snapshots are immutable and are never changed. Allocating them
with a lock is wasteful.

Review: https://reviewboard.asterisk.org/r/4469/
........

Merged revisions 432742 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432743 65c4cc65-6c06-0410-ace0-fbb531ad65f3