AS path filtering performance

[digizeph]

Currently, the filter for AS path is implemented as a String, and it needs to create a regular expression struct on every single path to do the matching, which is very much inefficient. We should refactor the internal storage of AS path filter to be constructed regular expression structs, and use re object to match on AS paths.

Besides, the matching also done based on AS path strings, which needs to be contrstuctred on every path, potentially very expensive as well. But I am unsure if there is a better way to do this.