Added global parameter server_priority to support in the selection of the server during token/fallback handling. If you have a multiple asterisk servers and are using fallback=true, then set server_priority in order or presedence per server.

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4984 43c1dc8c-776e-4df2-8ef1-6e829299be21