Convert Dynamic Features Buffer To Use ast_str

Currently, the buffer for the dynamic features list is set to a fixed size of
128.  If the list is bigger than that, it results in the dynamic feature(s) not
being recognized.

This patch changes the buffer from a fixed size to a dynamic one.

(closes issue ASTERISK-20680)
Reported by: Clod Patry
Tested by: Michael L. Young
Patches: 
    asterisk-20680-dynamic-features-v2.diff 
                                     uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2221/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@377915 65c4cc65-6c06-0410-ace0-fbb531ad65f3