BackendListener improvements

**sergiy.iampol** ([Bug 63486](https://bz.apache.org/bugzilla//show_bug.cgi?id=63486&redirect=false)):
It would be awesome having following available for BackendListener:

- AssertionMessage written to influx/graphite (as response message is not always enough);
- Ability to configure grouping for response messages and assertion messages to be written. (a set of regex which will be used to group above messages based on defined pattern)
For example:
"User TEST1 has a bad password"
"User GUEST2 has a bad password"
are about the same error type, but they will produce separate records on timeseries DB.

Having {"User \w+ has a bad password", "User xxx has a bad password"} regex as a BackendListener's pre-processor could combine such errors and produce single record with "User xxx has a bad password" as a message.

OS: