Use regular expression syntax in SaltEvent.get_event() for tag matching

Hi,

This is a continuation of https://github.com/saltstack/salt/pull/19077
(See discussion there)

Added a cache for regexes (salt.utils.cache.CacheRegex):
  * Default 1000 entries and expires every hour

Match types:
  * startswith
  * endswith
  * find
  * regex

I'll add some more info later on (including test / performance code and results).