Improve performance of the ast_event cache functionality.

This code comes from svn/asterisk/team/russell/event_performance/.

Here is a summary of the changes that have been made, in order of both
invasiveness and performance impact, from smallest to largest.

1) Asterisk 1.6.1 introduces some additional logic to be able to handle
   distributed device state.  This functionality comes at a cost.
   One relatively minor change in this patch is that the extra processing
   required for distributed device state is now completely bypassed if
   it's not needed.

2) One of the things that I noticed when profiling this code was that a
   _lot_ of time was spent doing string comparisons.  I changed the way
   strings are represented in an event to include a hash value at the front.
   So, before doing a string comparison, we do an integer comparison on the
   hash.

3) Finally, the code that handles the event cache has been re-written.
   I tried to do this in a such a way that it had minimal impact on the API.
   I did have to change one API call, though - ast_event_queue_and_cache().
   However, the way it works now is nicer, IMO.  Each type of event that
   can be cached (MWI, device state) has its own hash table and rules for
   hashing and comparing objects.  This by far made the biggest impact on
   performance.

For additional details regarding this code and how it was tested, please see the
review request.

(closes issue #14738)
Reported by: russell

Review: http://reviewboard.digium.com/r/205/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@184339 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/strings.h

@@ -851,6 +851,29 @@ static force_inline int attribute_pure ast_str_hash(const char *str)
 	return abs(hash);
 }
 
+/*!
+ * \brief Compute a hash value on a string
+ *
+ * \param[in] str The string to add to the hash
+ * \param[in] hash The hash value to add to
+ * 
+ * \details
+ * This version of the function is for when you need to compute a
+ * string hash of more than one string.
+ *
+ * This famous hash algorithm was written by Dan Bernstein and is
+ * commonly used.
+ *
+ * \sa http://www.cse.yorku.ca/~oz/hash.html
+ */
+static force_inline int ast_str_hash_add(const char *str, int hash)
+{
+	while (*str)
+		hash = hash * 33 ^ *str++;
+
+	return abs(hash);
+}
+
 /*!
  * \brief Compute a hash value on a case-insensitive string
  *