- simplify and improve astmm by using thread storage instead of a dynamic

  allocation and free on every call of the function for preparing the string
  that will be appended.  Then, use the ast_dynamic_str() code instead of the
  open coded version that is appended to when waiting for it to be delivered.
- use for loops for list traversals
- convert the manager sessions list to use list macros
- use atomic operations for num_sessions and usecounts
- convert some defines to the equivalent enum


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@40783 65c4cc65-6c06-0410-ace0-fbb531ad65f3