Convert pbx_spool to use string fields instead of statically-sized buffers.

In tests run after making this conversion, I noticed an approximate 85% 
reduction in memory usage for call file processing.

Review: http://reviewboard.digium.com/r/168/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@179745 65c4cc65-6c06-0410-ace0-fbb531ad65f3