sorcery: Create sorcery instance registry.

In order to retrieve an arbitrary sorcery instance from a dialplan function
(or any place else) there needs to be a registry of sorcery instances.

ast_sorcery_init now creates a hashtab as a registry.

ast_sorcery_open now checks the hashtab for an existing sorcery instance
matching the caller's module name.  If it finds one, it bumps the 
refcount and returns it.  If not, it creates a new sorcery instance,
adds it to the hashtab, then returns it.

ast_sorcery_retrieve_by_module_name is a new function that does a hashtab 
lookup by module name.  It can be called by the future dialplan function.

res_pjsip/config_system needed a small change to share the main res_pjsip 
sorcery instance.

tests/test_sorcery was updated to include a test for the registry.

(closes issue ASTERISK-22537)
Review: http://reviewboard.asterisk.org/r/3184/
........

Merged revisions 408518 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408519 65c4cc65-6c06-0410-ace0-fbb531ad65f3